sludge accumulation, characteristics, and pathogen ... · sludge accumulation, characteristics, and...

Water Research 38 (2004) 111–127

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doi:10.1016/j.w

Sludge accumulation, characteristics, and pathogeninactivation in four primary waste stabilization ponds in

central Mexico

Kara L. Nelsona,*, Blanca Jim!enez Cisnerosb, George Tchobanoglousc,Jeannie L. Darbyc

aDepartment of Civil and Environmental Engineering #1710, University of California, Berkeley, CA, 94720-1710, USAb Hydraulic and Environmental Engineering, Institute of Engineering, National Autonomous University of Mexico, CD Universitaria,

A.P. 70-472, Coyoacan 04510 Mexico, DFc Department of Civil and Environmental Engineering, 1 Shields Avenue, University of California, Davis, CA, 95616, USA

Received 8 May 2002; received in revised form 14 August 2003; accepted 8 September 2003

Abstract

To support the development of safe and feasible sludge management strategies, the accumulation rates of sludge and

its characteristics were studied in four primary wastewater stabilization ponds (WSPs) in central Mexico (three

facultative and one anaerobic). The accumulation rates and distribution of sludge were determined by measuring the

thickness of the sludge layer at 8–40 locations throughout each pond. The average, per capita sludge accumulation rates

ranged from 0.021 to 0.036 m3/person/yr. In the anaerobic pond the sludge distribution was uniform throughout the

pond, whereas in the three facultative ponds most of the sludge accumulated directly in front of the inlet. To measure

the horizontal and vertical variation in the sludge characteristics, sludge cores were collected from 3 to 7 locations in

three of the ponds. Each core was divided into 4 sub-samples in which various physical, chemical, and microbiological

parameters were measured. In addition, the inactivation of several pathogen indicator organisms was studied in a batch

of sludge for 7 months. Based on the microbiological results, it is concluded that reasonable estimates of the

inactivation of fecal coliform bacteria, fecal enterococci, F+ coliphage, somatic coliphage, and Ascaris eggs in WSP

sludge in central Mexico can be made using first-order rate constants of 0.1, 0.1, 0.01, 0.001, and 0.001 d�1, respectively.

From the observed changes in the concentrations of total solids and the volatile to fixed solids ratio, empirical equations

were developed to describe anaerobic degradation and compression, which are the two most important processes

affecting the volume of sludge after its deposition.

r 2003 Elsevier Ltd. All rights reserved.

Keywords: Waste stabilization pond; Wastewater stabilization pond; Sludge accumulation rate; Sludge distribution; Biosolids;

Pathogens

1. Introduction

Wastewater stabilization ponds (WSPs) are a simple,

low-cost, low-maintenance process for treating waste-

ing author. Tel.: +1-510-643-5023; fax: +1-

ess: [email protected] (K.L. Nelson).

e front matter r 2003 Elsevier Ltd. All rights reserve

atres.2003.09.013

water. A typical system consists of several constructed

ponds operating in series; larger systems often have two

or more series of ponds operating in parallel. Treatment

of the wastewater occurs as constituents are removed by

sedimentation or transformed by biological and chemi-

cal processes. In the bottom of the ponds, a sludge layer

forms due to the sedimentation of influent suspended

solids as well as algae and bacteria that grow in the

d.

ARTICLE IN PRESSK.L. Nelson et al. / Water Research 38 (2004) 111–127112

pond. Sludge accumulation is greatest in primary ponds

and can impact performance by altering the pond’s

hydraulics due to a decrease in the pond’s effective

volume and changes the shape of the bottom surface [1].

Therefore, periodic sludge removal is usually required

and the long-term sustainability of WSP systems is

dependent on the safe and effective management of their

sludge.

Despite the inevitable accumulation of sludge in

primary ponds, sludge management is rarely considered

as an integral part of pond design. One reason for the

lack of attention given to sludge is that little information

is available on the accumulation rates, the distribution

of sludge within the ponds, and the characteristics of the

sludge itself. The accumulation rate of sludge must be

known so that the frequency of sludge removal can be

determined and integrated into the pond design,

maintenance schedule, and budget. Currently, the most

common method for estimating sludge accumulation is

the empirically determined volumetric, per capita

accumulation rate [2–4]. However, the rates currently

recommended for design have not been widely validated,

and are believed to depend on temperature, among other

factors. Thus, more regional data are needed to

determine reasonable values for the per capita accumu-

lation rate until models based on pond characteristics

are fully developed [5–7].

In addition to knowing the rate of sludge accumula-

tion, it is necessary to know how the sludge is distributed

in a pond. The sludge distribution can have a significant

impact on the pond’s hydraulics (and consequently the

treatment efficiency), the frequency of sludge removal,

and the feasible options for sludge removal. The

distribution of sludge is primarily a function of the

pond configuration [8]. A better understanding of the

sludge distribution in ponds could lead to improvements

in design to achieve optimal distribution of the sludge.

More information is also needed on the characteristics

of the sludge itself. The sludge volume and character-

istics change with time due to anaerobic degradation,

compression, and pathogen inactivation; however, little

information has been published about the rates of these

processes and the typical characteristics of WSP sludge.

Data on the concentrations of pathogens in the sludge

layer are needed to estimate the risk that pathogens pose

upon removal of the sludge. Information on how the

sludge characteristics vary throughout the sludge layer is

also needed so that improved models of sludge

accumulation as well as sampling protocols for mon-

itoring programs can be developed.

The research reported herein was conducted in

Mexico, where WSPs are the most common type of

wastewater treatment and over 400 systems have been

built, most of them since 1980 [9]. To date, however, no

studies have been reported on the accumulation rates

and characteristics of sludge in Mexican ponds.

Furthermore, the removal of sludge has been under-

taken in only a few of the ponds, thus, information is

needed to support the development of a sludge manage-

ment plan and to prevent pond failures [10,11].

The goal of this research was to gather more

information on the sludge layer in WSPs to inform

improvements in pond design and support the develop-

ment of safe, effective sludge management practices. The

specific objectives were to:

1. Measure the accumulation rate and distribution of

sludge in four Mexican WSPs.

2. Evaluate the physical, chemical, and microbiological

characteristics of the sludge and their horizontal and

vertical variation within the sludge layer, including

characterization of compression, anaerobic degrada-

tion, and pathogen inactivation.

2. Experimental design and methods

Four primary WSPs located in the central highland

region of Mexico (B2500 m) were selected for this

research (Table 1). Pond depth and the thickness of the

sludge layer were measured in all ponds to determine the

sludge distribution and average accumulation rate.

Sludge cores were extracted from three of the ponds

(excluding San Jose de los Laureles); each core was

divided into four stratified sub-samples and various

physical, chemical, and microbiological parameters were

measured. In addition, a batch of sludge from one of the

ponds was stored in the laboratory for 7 months and the

concentration of pathogen indicator organisms was

measured periodically to gather more detailed data on

their rates of inactivation. The methods used to collect

field samples and the parameters measured are described

below.

2.1. Characteristics of field sites

The general characteristics of the anaerobic pond and

the three facultative ponds are presented in Table 1. The

pond in Texcoco treated wastewater from Mexico City,

whereas the other three ponds treated wastewater from

small communities. The degree of pretreatment varied

among the four pond systems. In both Mexicaltzingo

and San Jose de los Laureles, the wastewater passed

through a biodigester, in which some suspended solids

were removed by sedimentation, before entering into the

primary pond. In Texcoco, although the only formal

pretreatment was a bar screen and grit chamber, the

sewer canal that transported wastewater to the pond had

an insufficient slope to prevent settling, thus, significant

sedimentation of suspended solids occurred before the

wastewater was introduced to the primary pond. In

Xalostoc, the only pretreatment was a bar screen and

ARTICLE IN PRESS

Table 1

Characteristics of the four WSPs

Locationa Population Pond

type

Pretreatment Operation

period

(yr)

Qb,

Mgal/d

(L/s)

Surface

areab,

acres (ha)

Pond

depth,

ft (m)

Design

HRTc,

(d)

Average air temperature, �F (�C)d

Annual Coldest

month

Warmest

month

Mexicaltzingo, 7280 Primary Biodigester 5 0.34 0.14 8.9 2.5 54 49 58

Mexico anaerobic (15) (0.06) (2.72) (12.2) (9.4) (14.4)

San Jose de 800 Primary Biodigester 6 0.023 0.13 5.4 24 70 64 75

los Laureles, Facultative (1) (0.05) (1.64) (21.1) (19.1) (23.6)

Morelos

Texcoco, NAe Primary Settling in 10 2.15 6.0 5.1 10.6 60 55 65

Mexico Facultative sewer (94) (2.4) (1.56) (15.6) (12.9) (18.1)

Xalostoc, 11,000 Primary Grit 15 0.15 1.2 7.8 47 62 57 65

Tlaxcala Facultative chamber (6.4) (0.5) (2.39) (16.4) (13.9) (18.4)

aMunicipality, State.bFlow and area are only for the primary pond studied in this research (some systems had more than one primary pond operating in

parallel).cHRT=hydraulic residence time=Q/V, where V accounts for the wall slope and is therefore less than the volume calculated directly

from the values of surface area and pond depth shown in the table.dData are mean temperatures, 1951–1980, for the capital city of each state. Source: ‘‘Temperatura media mensual 1951–1980’’,

Servicio Meterol !ogico Nacional, Mexico.eNA=Not available. Because the wastewater treated in Texcoco is diverted from a large trunk sewer the population served is not

known.

K.L. Nelson et al. / Water Research 38 (2004) 111–127 113

grit chamber. At the time of sampling, all four ponds

had been in continuous operation for at least 5 years.

2.2. Measurement of pond depth and thickness of sludge

layer

Pond depth and sludge thickness were measured at

between 8 and 40 points in each pond with a sludge

measuring optical gauge (SMOG, Orenco Systems Inc.,

Sutherlin, OR, USA). The apparatus consisted of a

graduated pole with a detection unit at the end on which

a light source and light sensor were fixed 1 in apart. An

indicator light connected to the light source was on

when the detection unit was in the water column and

turned off when it entered into the sludge layer.

In San Jose and Xalostoc, a rectangular grid was

established around the perimeter of the ponds with

flagged stakes and each sampling point was located by

sighting off the flags. The Texcoco pond was too large to

locate the sampling points by visual sighting, so a

Global Positioning System (Garmin, Olathe, KS, USA)

was used. The sludge thickness data reported for the

Mexicaltzingo pond were obtained by another research

group using the white towel test [12].

Three-dimensional surface profiles of the sludge

distribution in each pond were created with a surface

mapping program (Surfer Version 7.00, Golden Soft-

ware, Inc., Golden, Co.). Grid files were generated from

each data set by the point kriging method using exact

interpolation (no smoothing). The bottom surface was

inferred from variations in the water depth. Surfer was

also used to calculate the total sludge and water volumes

for each pond by interpolating the sludge surface using

Simpson’s 3/8 rule and integrating. The volumes were

then corrected for irregular pond geometry (non-

rectangular) and for wall slope. Apparent sludge

accumulation rates (mm/yr) were calculated for each

pond by dividing the total sludge volume by the pond’s

bottom area and the number of years of operation.

2.3. Development of pneumatic apparatus for collecting

sludge cores

Sludge cores were collected from a rowboat at three to

seven locations in each pond, including near the

entrance, middle, and exit (Fig. 1). A sludge coring

apparatus was developed specifically for this research

because commercially available coring devices were

found to be inadequate. Typically, coring devices used

to sample lake, river, and ocean sediments have a

sediment catch that is forced open as the column is

pushed through the sediment. The pond sludge, how-

ever, was not dense enough to force open the sediment

catch without causing significant vertical disturbance of

the sludge layers. Also, such corers are primarily

designed to retain sediment and do not completely seal

at the bottom after the core is collected, allowing liquid

to drain out by gravity. Because the parameters

ARTICLE IN PRESS

(a)

(b)

(c)

(d)

Fig. 1. Sludge distribution in the four ponds (a) Mexicaltzingo (raw data collected by Lloyd et al. [12]) (b) San Jose de los Laureles, (c)

Texcoco, and (d) Xalostoc. The horizontal locations of the inlet and outlet structures are indicated by arrows. The locations at which

sludge cores were collected are shown with crosses.

K.L. Nelson et al. / Water Research 38 (2004) 111–127114

ARTICLE IN PRESS

Fig. 2. Diagram of pneumatic apparatus used to collect sludge

cores (not to scale).


measured in this research were dependent on dissolved

constituents and very small particles, it was necessary to

completely seal the bottom of the core before it was

raised from the sludge layer.

Several previous researchers have used a small

diameter PVC pipe to obtain sludge cores from ponds;

the pipe was screwed into the compacted clay lining or

soil at the pond bottom to form a plug that sealed the

bottom of the core [13–16]. However, this method would

not work in a pond with an artificial liner, such as

concrete (Mexicaltzingo) or a geomembrane (San Jose

de los Laureles). Furthermore, the ability of the plug to

support the weight of the overlying sludge/water column

limits the pipe diameter, thereby limiting the sample size

as well as increasing the potential for contamination

of lower layers by upper layers as the small-diameter

pipe is pushed into the sludge. Thus, a rotating stainless

steel door activated by a pneumatic cylinder was

developed for this research to completely seal the core

even in very thick sediments (30% total solids) and at

water depths of 3 m without collecting material from the

pond lining; a pressurized air tank was necessary to

operate the corer.

A diagram of the coring apparatus is provided in

Fig. 2. Sample ports were located such that a minimum

of 4 sub-samples could be obtained from each sludge

core greater than 150 mm in height. Aluminum pipes

were attached to the top of the corer to permit sludge

collection at water depths greater than 0.8m. After a

core was collected, overlying water was removed using a

large syringe. A sample was also collected with the

syringe at the water–sludge interface. The remaining

sludge samples were collected in bottles starting with the

top layer by allowing the sludge to flow out the sampling

ports. A specially designed plunger was used to force

thick sludge out of the ports.

2.4. Estimation of sludge age

To investigate the effect of age on sludge character-

istics, a method for estimating the age of each sludge

core sub-sample was needed. Sludge age is similar to

relative depth, but while both parameters account for

spatial variation in the rate of deposition to the sludge

layer, only sludge age accounts for compression and

biodegradation. Thus, at each core location it was

assumed that fixed solids were deposited to the sludge

layer at a constant rate throughout the operational

lifetime of the pond, and that once in the sludge layer

they were conserved. The age of the sludge at any depth

was calculated to be directly proportional to the mass

fraction of fixed solids that had been deposited above

that depth, and the average age of each sample was

calculated by averaging the age at the top and the

bottom of the sample. The following equation was

developed:

Aj ¼

mass of fixed solids in sample

j and samples above it

total mass of fixed solids

in sludge core

years of operation

¼Pj

i¼1 FSiP4i¼1 FSi

" #Y ; ð1Þ

where j is the sub-sample of interest, j=1, 2, 3, 4, Aj

the age of sludge at bottom of sub-sample j, FSi the

mass of fixed solids in sub-sample i, i=1,2,y,j,

and Y is the number of years the pond had been in

operation.

2.5. Measurement of horizontal and vertical changes in

sludge layer characteristics

In each sludge core sub-sample, several physical,

chemical, and microbiological parameters were mea-

sured (Table 2). Temperature, pH, and oxidation

ARTICLE IN PRESS

Table 2

Description of the parameters measured and the methods used in the sub-samples from the sludge cores

Parameter Methoda Ponds sampled

Temperature Temperature probe Mexicaltzingo,

pH pH electrode Texcoco, and

Oxidation reduction potential (ORP) Platinum ORP electrode Xalostoc

Total solids (TS) Gravimetric (2540 Gb) ‘‘

Volatile solids (VS) ‘‘ ‘‘

Fixed solids (FS) ‘‘ ‘‘

VS/FS ‘‘ ‘‘

Helminth eggs US EPA, [17] ‘‘

Fecal coliform Multiple tube fermentation, direct method (9221 E.2) ‘‘

Fecal enterococci Spread plate (9215 C) with mEnterococcus agar (9230 C.2.c) Xalostoc only

Somatic coliphage Double agar layer [18], naldixic acid resistant E.coli CN 13 ‘‘

F+ coliphage Double agar layer [18], streptomycin/ampicillin—resistant E. coli Famp ‘‘

aNumbers in parenthesis refer to Standard Methods [19].bA 50-mL syringe with the tip cut off was used to measure the initial sample volume to allow measurement of the solids’

concentrations on a weight per volume (w/v) as well as a weight per weight (w/w) basis.


reduction potential (ORP) were measured immediately

after extrusion from the core. Then, the samples were

transported in a cooler to the laboratory and stored at

4�C. Total, volatile, and fixed solids, as well as all

microbiological analyses, except helminth eggs, were

performed on well-homogenized samples within 24 h of

sample collection. Determination of helminth eggs was

performed within one week of sample collection in most

cases; no significant change in the viability of the eggs

was expected during this time because the eggs can

survive for years at low temperatures [20]. Care was

taken during enumeration of the helminth eggs to

minimize exposure to diethyl ether [21].

Fecal coliform bacteria and helminth eggs were

enumerated in all sludge core samples. In the Xalostoc

pond, three additional indicator organisms were mea-

sured—fecal enterococci, somatic coliphage, and F+

coliphage. For analysis of fecal coliform bacteria and

fecal enterococci, the first dilution consisted of blending

10 g of sample with 90 mL of buffered dilution water

(9050C 1a [19] at high speed for 1min. For enumeration

of somatic and F+ coliphages, the host bacterial strains

were E. coli CN13 (resistant to naldixic acid) and E. coli

Famp (resistant to streptomycin and ampicillin), respec-

tively (obtained from Dr. Mark Sobsey, Dept. of

Environmental Science and Engineering, University of

North Carolina). To prepare samples for coliphage

analysis, an initial elution was performed by vigorously

mixing 5 g of sample with 5mL of 0.1% Tween 80 for

1min in a 50-mL centrifuge tube. The sample was

left to sit for 10min and mixed again for 1min.Then,

5mL of chloroform was added, the sample was mixed

vigorously for 3min, and centrifuged for 20 min at

2000� g. Serial dilutions were prepared from the

supernatant.

2.6. Measurement of indicator organism inactivation in

batch test

Approximately 3 L of sludge was removed from the

surface of the sludge layer near the inlet of the Xalostoc

pond. The sludge was stored in the laboratory at

ambient temperature in a 4L container; the lid was

closed, but not sealed, to allow gas to escape.

Concentrations of fecal coliform bacteria, fecal enter-

ococci, somatic coliphage, and F+ coliphage were

measured periodically for 7 months. The enumeration

methods were the same as those used for the sludge

cores. Over the duration of the batch experiment, the

temperature in the sludge ranged from 11�C to 16�C,

which was similar to the range of temperatures

measured in the Xalostoc sludge layer.

3. Results and discussion

3.1. Sludge distribution and rate of accumulation

In all three facultative ponds, the distribution of

sludge was very uneven (Figs. 1b–d), whereas in the

anaerobic pond it was fairly uniform (Fig. 1a). In the

facultative ponds, the maximum sludge thickness

occurred near the single pond inlet; higher accumulation

also occurred in some of the corners. In the anaerobic

pond, the more even sludge distribution was attributed

to two factors. First, instead of one inlet the pond had

five, so the incoming solids were distributed over a larger

surface area. Second, the hydraulic residence time

(HRT) was much shorter (2 d compared with more than

11 d), thus, the overflow rate in the pond was much

ARTICLE IN PRESSK.L. Nelson et al. / Water Research 38 (2004) 111–127 117

higher and solids were carried further into the pond

before settling to the bottom.

In this research, from 8% to 25% of the ponds’

volumes were occupied by solids, resulting in propor-

tional decreases in the design HRT (Table 3). It is likely

that the effective HRTs in the facultative ponds were

even further reduced by the formation of preferential

flow paths and dead zones. The results from this re-

search contribute to a growing body of evidence demon-

Table 3

Sludge thickness, volume, and resulting decrease in the hydraulic rete

Pond System Operation

period, (yr)

Sludge thickness (m)

Avg. Max.

Mexicaltzingoa 5 0.67 0.81

San Jose 6 0.15 0.38

Texcoco 10 0.36 1.11

Xalostoc 15 0.34 1.95

aRaw data collected by Lloyd and Vorkas [12].

Table 4

Mean concentrations of solids in the sludge layer and measured accum

reported in the literaturea

Pond location Pond type No. of

ponds

sampled

Operation

period (yr)

Mean TS

(g/L)

This research

Mexicaltzingo Anaerobic 1 5 171

San Jose Facultative 1 6 NA

Texcoco Facultative 1 10 112

Xalostoc Facultative 1 15 166

Literature values

Columbia Anaerobic 2 2.6 NA

5

SE Brazil Anaerobic 2 NA 172

NA

NE Brazil Facultative 1 2.5 39

France NA 1 10 187

France Facultative 12 3–10 54–136

Mississippi,

USA

Facultative 15 0.5–7 35–192

Utah, USA Facultative 2 7 59

13 77

aNA=not available.bBecause the wastewater treated in Texcoco was diverted from a lacValues were calculated by averaging data reported for 15 sludge cdValue was calculated using raw data reported for two sludge coreeValues for solids concentrations were presumably calculated fromfData are averages of sludge cores collected along two perpendicugSampling protocol accounted for variation of sludge characteristi

strating that in facultative ponds with single inlets, the

majority of sludge accumulates directly in front of the

inlet [8,13,22,23]. More information is needed on alter-

native inlet configurations that would distribute the

sludge over a larger area, such as installing additional

inlet pipes or increasing the inlet velocity or direction.

The sludge accumulation rates were determined both

on a per capita basis and as the average annual net

increase in sludge thickness (Table 4), because both are

ntion times (HRT) of the four ponds

Sludge

vol./total

vol(%)

HRT without

sludge (d)

HRT with

sludge (d)

Min.

0.61 25.3 2.5 1.9

0.05 8.2 24 22

0.06 14.4 10.6 9.0

0.07 13.2 47 41

ulation rates in the ponds from this research compared to those

Mean

VS/FS

Accumulation rate Reference

m3/pers yr mm/yr

0.63 0.022 119

NA 0.036 21

0.57 NAb 21

0.67 0.021 19

NA 0.055 NA Pena et al. [1]

0.040

0.62 0.023 77 Teles et al. [24]

NA 0.026 53

1.5 NA NA Ayres et al. [25]c

NA NA NA Carr!e and Baron [15]d

0.29–

0.94

0.12 15–85 Carr!e et al. [23]e

0.11–

0.59

NA 15–51 Middlebrooks et al. [13]f

2.23 NA 6.85 Schneiter et al. [16]g

4 8.1

rge trunk sewer the population served is not known.

ores collected throughout the pond.

s, one near the inlet and one near the outlet.

sludge cores.

lar axes in each pond.

cs with location and depth.

ARTICLE IN PRESS

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

MexicaltzingoTexcocoXalostoc

Rel

ativ

e de

pth

Relative age

Fig. 3. Relationship between relative depth in the sludge layer

and relative sludge age in the three ponds.


used in design. The average, per capita accumulation

rates ranged from 0.021 to 0.036m3/person/yr in the

Mexican ponds; these rates are similar to values that

have been measured in Columbia and Brazil but are

significantly lower than rates measured in France. The

higher accumulation rates in France may be partly due

to colder temperatures, although this cannot be con-

firmed because the locations and temperatures of the

French ponds were not reported. Other factors affecting

the per capita sludge accumulation rate may include

sewer inputs by industry, stormwater, and infiltration.

Although published values determined from field data

are few, a value of 0.04 m3/person/yr is often recom-

mended for designing anaerobic ponds with average

temperatures above 20�C [2,3,26,27]. It is concluded

from the results of this research that 0.04m3/person/yr is

a reasonable estimate of the average rate of sludge

accumulation in both facultative and anaerobic ponds in

the central region of Mexico, even in regions with

average temperatures well below 20�C. Additional field

data on accumulation rates are needed from different

regions and pond types.

In contrast to the per capita rate of sludge accumula-

tion, the measured average annual increase in sludge

thickness varied widely, both in the ponds from this

research (19–119 mm/yr) and among values reported in

the literature (7–85mm/yr) (Table 4). This variation is

expected because the depth accumulation is strongly

affected by the pond loading rate and treatment

efficiency and is thus specific to each pond. For example,

although the per capita accumulation rates were similar

in the Mexicaltzingo and Xalostoc ponds, the thickness

of the sludge layer increased over five times faster in

Mexicaltzingo because the detention time was much

shorter.

3.2. Horizontal and vertical variation of sludge

characteristics

The horizontal variation of sludge characteristics was

studied by comparing the average values of each

parameter in sludge cores collected near the inlet,

middle, and outlet of each pond. For studying the

vertical variation, the appropriate independent variable

was either depth within the sludge layer or sludge age,

depending on the parameter. The total solids concentra-

tion was found to be dependent on the depth within the

sludge layer, whereas the remaining parameters were

either dependent on the sludge age or constant

throughout the sludge layer.

3.2.1. Estimation of sludge age

The relationship between depth in the sludge layer

and the estimated sludge age (Eq. (1)) is shown in Fig. 3.

To account for differences in the operation periods and

the thickness of the sludge layer, the normalized values

are plotted. If the sludge depth had increased by the

same amount each year, then the relationship would be a

straight line with a slope of one. However, as shown in

the figure, the newer sludge occupied more volume than

the older sludge. The figure can also be interpreted as

showing the increase in sludge depth with time; the slope

then represents the relative accumulation rate, which

decreases with time.

Potential sources of error in estimating the sludge age

include changes in the concentration of fixed solids in

the influent wastewater, production rate of algae and

bacteria, hydraulic flow pattern, sludge settling zones,

and resuspension or lateral movement of sludge over the

operational lifetime of the pond. Also, the production of

fixed solids through new cell growth in the sludge layer

was not accounted for. Historical data were not

available for any of the ponds, but according to

interviews with plant operators and municipal autho-

rities, no major changes occurred in any of the sewer

networks that fed the ponds since operation was

initiated, nor were there periods when the ponds were

non-operational.

3.2.2. Total solids concentration

The total solids (TS) concentration was found to be

correlated with depth in the sludge layer (R2=0.84),

with values increasing from around 3 g/L at the sludge/

water interface to over 300 g/L in the deepest sludge

(Fig. 4a). It is a significant finding that the compression

of sludge solids appeared similar throughout all three

ponds. Furthermore, no relationship was found between

TS and variables describing the qualitative character-

istics of the sludge, such as age or VS/FS ratio. The

resulting regression equation can be used to estimate the

ARTICLE IN PRESS

0

50

100

150

200

250

300

350


TS

, g/L

TS, g/L = 417(Depth, m)0.57

0

5

10

15

20

25

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Tem

pera

ture

, °C

Inlet Middle OutletDepth in sludge layer, m Location in pond

(a) (b)

(c) (d)

Fig. 4. Profiles of the total solids concentration (a and b) and temperature (c and d) as a function of depth in the sludge layer and

distance from the pond inlet. In (a), the data points represent individual sub-samples from the sludge cores, whereas average values are

reported in (b), (c), and (d).


solids concentration of the sludge if the thickness of the

sludge layer is known. This information is needed to

develop more accurate methods for estimating the rate

of sludge accumulation, because the volume (and

therefore depth) occupied by a given mass of sludge is

a function of the TS concentration (which is an

approximate measure of the sludge density). Knowing

the TS concentration may also help to determine the

most appropriate method of sludge removal.

In the facultative ponds, the concentration of total

solids (TS) decreased towards the pond outlet, whereas

the concentration in the anaerobic pond was similar

throughout (Fig. 4b). The higher TS concentration near

the inlet in the facultative ponds is primarily a reflection

of the greater thickness of the sludge layer, which causes

more compression, but may also be affected by a higher

fraction of higher density silts and sand that settle out

near the inlet. The mean TS concentrations in the ponds

from this research are reported in Table 4, and fall

within the upper limit of values that have been reported

in the literature. In general, the low TS values that have

been reported occurred in sludge with a high concentra-

tion of volatile solids (high VS/FS ratio).

3.2.3. Temperature

At the time the ponds were sampled, the temperature

was fairly constant throughout the sludge layer (Figs. 4c

and d), although it was usually significantly different

from the temperature in the overlying water layer and

the air temperature (not shown). Over the course of one

sampling day, the water temperature often varied more

than 5�C while the temperature of the sludge layer

remained constant. The average air temperatures during

the months that the ponds were sampled were 10.8�C,

14.8�C, and 18.3�C in the Mexicaltzingo, Texcoco, and

Xalostoc regions, respectively. There was much less

variation in the average temperatures in the sludge layer

of the ponds, which were 16.3�C, 16.7�C, and 17.9�C,

respectively. The buffering of the sludge temperature can

be explained because the rate of heat transfer depends

on the degree of mixing, which is expected to be lower in

the sludge than in the water or air. It is important to

point out that the similarity in average sludge tempera-

tures in the three ponds at the time of sampling was

coincidental, and does not indicate that the sludge

temperatures were similar at other times of the year. The

Xalostoc pond, for example, was sampled again 3

months after the initial sampling and the sludge

temperature had dropped to 14.3�C (the average air

temperature dropped to 14.1�C). More research is

needed on the fluctuations of temperature in the sludge

layer so that the implications on temperature-dependent

microbial processes, such as anaerobic degradation and

pathogen inactivation, can be defined.


3.2.4. Anaerobic degradation (VS/FS, pH, and ORP)

The ratio of volatile to fixed solids (VS/FS) was used

as an approximate measure of the fraction of organic to

inorganic matter in the WSP sludge. The decrease in the

VS/FS ratio with sludge age was similar in all three

ponds, and the slope of the line was interpreted as the

rate of anaerobic degradation (Fig. 5a). The initial

degradation rate was several orders of magnitude

greater than the long-term degradation rate, parti-

cularly in cores with a high initial VS/FS ratio

(individual core data not shown). It is believed that the

decrease in degradation rate occurred as the rapid

degradation of easily hydrolyzable organic matter

was replaced by the continual, slow degradation of

recalcitrant organic matter that required considerable

0

0.25

0.5

0.75

1

1.25


VS

/FS

5

6

7

8

9

pH

-200

-150

-100

-50

0

50

0 2 4 6 8 10 12 14

OR

P, m

V

Estimated sludge age, yrs

(a)

(c)

(e)

Fig. 5. Profiles of the volatile to fixed solids ratio (a and b), pH (c and

sludge age and distance from the pond inlet.

processing before it could be hydrolyzed and

degraded by anaerobic bacteria. This trend has been

identified previously, although not quantitatively

[15,28,29].

Within the first year, about 30% of the VS was

degraded in the facultative ponds and about 25% in the

anaerobic pond, on average, although these figures may

be underestimated because it was difficult to make an

accurate measure of the initial VS/FS ratio. Better

methods for collecting fresh sludge are needed so that

the decrease in VS during the first few weeks after

deposition can be quantified [29].

The long-term, first-order inactivation rate constants

(after the first year) were determined from the slope of

the linear regression fitted to the log-transformed VS/FS

Inlet Middle Outlet

Location in pond

(b)

(d)

(f)

d), and oxidation reduction potential (e and f) as a function of


data, discarding the initial measurement. The values

were 0.122, 0.061, and 0.042 yr�1 in the Mexicaltzingo,

Texcoco, and Xalostoc sludge, respectively. These values

are believed to be the first reported on the long-term

degradation rates in WSP sludge, and are useful for

estimating the reduction in sludge mass with long-term

storage in the pond. In all three ponds the VS/FS ratio

in the oldest sludge was approximately 0.5, suggesting

that some organic matter may not degrade for many

years, if ever.

In all of the ponds, the VS/FS ratio increased towards

the outlet (Fig. 5b). A possible explanation is that the

denser, inorganic solids settled closer to the inlet,

whereas the lighter, organic solids settled out near the

outlet (predominately bacteria and algae in the faculta-

tive ponds). A similar increase in the VS/FS ratio

towards the outlet was observed in a primary facultative

WSP in northeast Brazil [25]. The mean VS/FS ratio in

the ponds from this research is reported in Table 4; the

values are also compared to others reported in the

literature.

The pH was constant throughout the sludge layer in

all three ponds, with average values between 6.8 and 6.9

(Figs. 5c and d). The neutral pH suggests that

methanogenesis was occurring, otherwise a build-up of

fatty acids, the products of acidogenesis, would even-

tually overcome the buffer capacity of the sludge and

cause the pH to drop. The transport of fatty acids to the

overlying water column may also help to maintain stable

conditions in the sludge layer.

Although the neutral pH measured throughout the

sludge layer suggests that methanogenesis was occur-

ring, the oxidation reduction potentials (ORP) measured

in the sludge layer suggested the dominance of sulfate-

reducing reactions (Fig. 5e). However, ORP was

measured in the sludge samples after extrusion from

the sludge corer, thus, it is possible that the measured

values were higher than the actual values due to oxygen

exposure upon sampling. In addition, the 2-mm

platinum electrode may have been too large to detect

microsites in which methanogenesis occurred. In future

studies, it is suggested that ORP be measured directly in

the undisturbed sludge layer, with a smaller electrode.

The values measured in this research are higher than

those reported by Carr!e et al. [15] for pond sludge in

France, in which minimum values of –300 mV were

found. In both of the facultative ponds the ORP was

positive in the overlying water, as expected, and dropped

sharply at the surface of the sludge layer. In contrast, in

the anaerobic pond the ORP in the overlying water was

similar to the ORP at the surface of the sludge layer. No

significant change in oxidation reduction potential

(ORP) was observed in the facultative ponds from the

inlet to the outlet (Fig. 5f); no comparison could be

made in the anaerobic pond because measurements were

taken at only one location.

3.2.5. Helminth eggs

The concentration of total helminth eggs was constant

with sludge age in Xalostoc, but increased with age in

Mexicaltzingo and decreased in Texcoco (Fig. 6a).

Apparently, fewer eggs were deposited in the sludge

layer during recent years in Mexicaltzingo. In Texcoco,

on the other hand, the most likely explanation for the

observed trend is that eggs were physically destroyed in

the aging sludge because the percentage of total eggs

that were viable actually increased with sludge age,

whereas in the other two ponds it decreased (Fig. 6c).

One reason that eggs could have been destroyed in

Texcoco and not in the other two ponds is that the

Texcoco pond treated wastewater from Mexico City,

whereas the other two ponds treated wastewater from

small communities. Thus, compounds could have been

present in the Texcoco sludge, such as from industrial

discharges, that caused a destruction of the eggs; the

high salinity of the Texcoco wastewater may also have

affected the eggs.

The concentration of helminth eggs decreased

dramatically towards the outlet of the facultative

ponds, but was similar throughout the anaerobic pond

(Fig. 6b). This pattern reflects the settling conditions

in the ponds; the concentration of helminth eggs

follows approximately the distribution of sludge. The

percentage of viable helminth eggs also decreased

significantly towards the outlet in the facultative

ponds; the reported values are for the sub-samples

from the top of the cores (Fig. 6d). A similar pattern

was observed in a primary pond in northeast Brazil [30].

One reason for this trend could be that non-viable

eggs have a lower density than viable eggs and there-

fore take longer to settle out (the density was not

measured, however, in this research). Another reason

could be that older, inactivated eggs were resuspended

and a net movement of sludge toward the outlet

occurred.

More than 85% of the helminth eggs isolated from the

sludge were Ascaris sp.; the remaining eggs were

Trichuris, Hymenolepis, and Toxocara. sp. (Table 5).

The species distribution is a function of the prevalence

of infection in the community, as well as the settling

patterns in the pond. Only the inactivation rate of

Ascaris is considered here, because the eggs of Ascaris

were more resistant than those of Trichuris and

Toxocara (data not shown), and because the viability

of Hymenolepis could not be determined by the method

used. To measure the inactivation rate, it was assumed

that no transport of eggs occurred within the sludge

layer. A change in the concentration of viable Ascaris

eggs per gram fixed solids was chosen as the most

accurate measure of inactivation; basing the concentra-

tion on fixed rather than total solids eliminated any bias

introduced by degradation of the sludge. Although it

was expected that the inactivation rate varied with

ARTICLE IN PRESS

0

100

200

300

400

500

600


Tot

al h

elm

inth

egg

s, #

/g F

S

0

20

40

60

80

100

Via

ble

helm

inth

egg

s, %

10-1

100

101

102

103

Via

ble

Asc

aris

egg

s, #

/g F

S

100

101

102

103

104

105

106

107

108

0 2 4 6 8 10 12 14

Fec

al c

olifo

rm b

acte

ria, M

PN

/g T

S


Inlet Middle Outlet

Location in pond

(a) (b)

(c) (d)

(e) (f)

(g) (h)

Fig. 6. Profiles of the concentration of total helminth eggs (a and b), percentage of viable helminth eggs (c and d), concentration of

viable Ascaris eggs (e and f), and concentration of fecal coliform bacteria (g and h) as a function of sludge age and distance from the

pond inlet.


location in the pond, with seasonal changes in tempera-

ture, and with the concentration of acids, ammonia, and

predatory microorganisms in the sludge layer [31], the

goal of this analysis was to determine a rough estimate

of the average inactivation rate constant throughout the

whole pond and over its entire lifetime.

ARTICLE IN PRESS

Table 5

Species composition (%) of helminth eggs in the sludge layer of

the three ponds

Pond Ascaris Trichuris Hymenolepis Toxocara

Mexicaltzingo 93.5 3.5 0.2 2.8

Texcoco 85.3 13 1.7 0.4

Xalostoc 87.4 2.2 9.8 0.6

Average 88.7 5.4 4.5 1.3

Table 6

First-order rate constants for the inactivation of Ascaris eggs

and indicator organisms. The values for the indicator organisms

were determined by two methods—sludge cores and a batch test

Organism k (d�1)

Sludge cores Batch test

Ascaris eggs

Mexicaltzingo 0.0009

Texcoco 0.0007

Xalostoc 0.0010

Indicator organisms (Xalostoc)

Somatic coliphage 0.0016 0.0074

F+coliphage 0.016 0.037

Fecal coliform 0.13 0.16

Fecal enterococci 0.26 0.20


The inactivation rate constants were determined

from the slope of the linear regression fitted to the

log-transformed concentration of viable Ascaris eggs

(Fig. 6e). The values for all three ponds were similar,

ranging from 0.0007 to 0.001 d�1 (Table 6). It is

concluded that a first-order rate constant of 0.001 d�1

is reasonable for estimating the average inactivation of

Ascaris eggs in the sludge layer of WSPs in central

Mexico. In a study reported separately, a similar mean

inactivation rate was measured in dialysis chambers

loaded with Ascaris eggs and stored in the Texcoco

sludge layer for 14 months [32]. These values are the first

known report of Ascaris egg inactivation rates in WSP

sludge.

3.2.6. Indicator organisms

In contrast to the helminth eggs (Fig. 6b), there was

no clear trend in the concentration of fecal coliform

bacteria (Fig. 6h) and the other indicator organisms

(data not shown) from the inlet to the outlet. However,

in all three ponds the concentration of fecal coliform

bacteria decreased with sludge age (Fig. 6g) and the

pattern was similar for somatic coliphage,1 F+coliph-

age, and fecal enterococci, although the rate of decrease

varied for the different organisms (Fig. 7a).

The inactivation rates of the four indicators were

determined from the sludge core data assuming first-

order kinetics based on the initial concentration of the

organisms at the sludge/water interface and the mean

concentration of organisms in each sludge core, because

these values were measured with a high-degree of

confidence. The relationship between the mean and

1At the time the experiment was initiated, beef extract was

not available for preparation of the phage eluent, so a solution

of 0.1% Tween 80 was used. At a later date, a comparison was

undertaken in which triplicate sludge samples were eluted using

either 0.1% Tween 80 or 3 g of beef extract in 0.1% Tween 80.

The concentrations of F+ and somatic phages were 40–50%

lower in the samples eluted with only Tween 80. Thus, the

concentrations of phages measured in this study are likely to be

at least 50% lower than the actual concentrations in the sludge.

However, it is believed that the underestimation of the actual

phage concentration did not have a significant effect on

calculation of the inactivation rates.

initial concentration is:

Cm ¼Co

R t1t0

e�kt dtR t1t0

dt¼

�Co=k e�kt1 � e�kto� �t1 � t0

; ð2Þ

where Cm is the mean (geometric) concentration of

organism measured in the entire sludge core, No./g FS,

Co the concentration of organism measured at the sludge

water interface, No./g FS, k the first-order inactivation

rate constant, d�1, t0 the age of sludge at top of core

(sludge/water interface), d, and t1 is the age of sludge at

bottom of core, d.

The concentration of indicator organisms in the batch

of sludge that was stored in the laboratory also

decreased with time (Fig. 7b). The inactivation rate

constants in the batch experiment were calculated by

fitting a straight line to the log-transformed concentra-

tions (No./g FS) of the four organisms, discarding

points in the tailing region. The constants calculated

from both the sludge cores and the batch experiment are

reported in Table 6. Based on the close agreement

between the two independent measures of inactivation, it

is concluded that values of 0.1, 0.01, and 0.001 d�1 are

conservative estimates of the first-order rate constants

for fecal coliform (and fecal enterococci), F+ coliphage,

and somatic coliphage, respectively, under the condi-

tions of this research.

Fecal coliform bacteria and fecal enterococci are

commonly used as indicators of enteric bacteria

inactivation, and F+ coliphage may be an adequate

indicator of enteric virus inactivation [33–36]. Thus, the

results from this study provide strong evidence that in

the sludge layer of WSPs in central Mexico, most enteric

bacterial pathogens are inactivated within several

months, whereas the inactivation of viral pathogens

may take several years.

ARTICLE IN PRESS

-5

-4

-3

-2

-1

0

0 2 4 6 8 10 12 14

Somatic coliphage

F+ coliphage

Fecal coliform bacteria

Fecal enterococci

Log

(C/C

o)Lo

g (C

/Co)


1.3 x 105

6.6 x 103

1.8 x 103

9.3 x 102

0 1 2 3 4 5 6 7 8-5

-4

-3

-2

-1

0

Time, months

4.0 x 105

1.9 x 103

1.4 x 1031.3 x 102

(a)

(b)

Fig. 7. Relative concentrations of indicator organisms mea-

sured in the (a) sludge cores and (b) a batch test of sludge from

Xalostoc. The final concentrations (organisms/g TS) are also

reported.


3.2.7. Contamination of sludge core samples

If Figs. 7a and b are compared, there is a clear

discrepancy between the concentrations of the four

indicator organisms remaining in the sludge cores and

those in the batch test (note the different time scales).

Based on the determined rate constants (Table 6), the

concentrations of the four indicator organisms measured

in the older sludge core sub-samples could not have been

due to surviving organisms; the most likely explanation

is that the older sludge was contaminated by the newer

sludge when the sub-samples were extruded from the

corer. Because the initial concentrations of organisms

were very high (106–107) and the inactivation rates rapid

compared to the age of the sludge, even a small degree of

contamination would have obscured the actual decrease

in concentration with sludge age. The method used to

calculate the inactivation rate in the sludge cores

(Eq. (2)), however, was not affected by the apparent

contamination because it was based on the mean

concentration of the entire sludge core.

In the case of helminth eggs, the apparent contamina-

tion would have had a minimal impact on the measured

concentrations because the relative change in concentra-

tion from the youngest to the oldest sludge was much

lower. Based on similar logic, it is reasoned that there

was no significant impact from contamination on the

remaining parameters measured in this research. It

should be emphasized, however, that when using sludge

cores, as with any field sampling method, the results

should be interpreted carefully.

3.2.8. Summary and implications for sludge management

The characteristics of the WSP sludge varied more in

the vertical than horizontal direction. In fact, in the

anaerobic pond there were no significant trends in the

horizontal direction, which is consistent with its even

distribution of sludge. Analogously, the parameters that

varied horizontally in the facultative ponds were those

that were a function of sedimentation (TS, VS/FS,

concentration and viability of helminth eggs), with the

denser constituents accumulating near the pond inlet

where the thickness of the sludge layer was greatest. In

all ponds, the sludge was transformed by compression,

anaerobic degradation, and inactivation of the micro-

organisms such that significant variation of the sludge

characteristics was observed in the vertical direction.

One implication of the observed horizontal and vertical

variations is that monitoring of the sludge layer cannot

be achieved by taking measurements in one location or

collecting grab samples. Rather, it is recommended that

a protocol similar to the one used in this research be

followed, in which sludge cores are collected from

several representative locations throughout the pond.

Within the first few months to 1 year in the sludge

layer, significant stabilization of the organic matter and

inactivation of helminth eggs and indicator organisms

occurred. Thus, a significant improvement in sludge

quality may be achieved if a pond is taken out of

operation for a period before the sludge is removed. This

option, however, requires that ponds are operated in

parallel such that the remaining ponds can accommo-

date 100% of the inflow. Although the rates are

expected to vary from region to region as a function

of temperature, if sludge is removed during normal pond

operation the recently deposited sludge will exert a

dominant influence on the sludge characteristics.

ARTICLE IN PRESS

Table 7

Meana and maximum concentrations of helminth eggs and indicator organisms in the sludge layers of the three ponds and maximum

values of helminth eggs and fecal coliform bacteria allowed by the USEPAa and Mexican governmentb in biosolids that are to be land

applied

Location Total

helminth

eggs,

eggs/g TS

Viable

helminth

eggs,

eggs/g TS

Somatic coliphage,

pfu/g TS

F+coliphage,

pfu/g TS

Fecal coliform

bacteria,

MPN/g

TS

Fecal enterococci,

cfu/g TS

Mean Max. Mean Max. Mean Max. Mean Max. Mean Max. Mean Max.

This research

Mexicaltzingo 129 184 25 55 1.3� 105 1.2� 107

Texcoco 49 273 25 169 5.7� 104 1.5� 107

Xalostoc 277 657 48 257 5.3� 105 4.2� 106 1.2� 104 1.3� 106 3.1� 104 4.4� 107 3.4� 103 7.9� 106

Regulationc

US EPA Class A 0.25d 1� 103

US EPA Class B No limite 1� 103

Mexico Class A 10 1� 103

Mexico Class B 35 2� 106

aGeometric mean for bacteria and virus, arithmetic mean for helminth eggs.bUSEPA [37].c INE [38]. Also stipulates a Salmonella concentration p3 and p300 MPN/g TS for Class A and B biosolids, respectively.dThe actual standard requires measurement of 4 g TS, such that the concentration is o1 viable egg/4 g TS.eNo regulations exist for the concentration of Somatic coliphage, F+coliphage, or Fecal enterococci.


Depending on the sludge removal process, evaluating

the risk posed by the pathogens in the sludge may

require determination of both the maximum and

average concentrations of the pathogens and/or indica-

tor organisms. The concentrations of helminth eggs and

fecal coliform bacteria measured in this research

exceeded the values allowed by the US EPA for both

Class A and B biosolids and the Mexican government

for Class A biosolids. The average values in the

Mexicaltzingo and Texcoco sludge met the Mexican

requirements for Class B biosolids (Table 7).

4. Conclusions and recommendations

The results from this research on the characteristics of

WSP sludge and the rates of the most important

transformation processes in the sludge layer—compres-

sion, anaerobic degradation, and pathogen inactiva-

tion—can be used to evaluate sludge removal and

treatment options. Based on the estimated degradation

and inactivation rates, a significant improvement in

sludge quality could be achieved by taking a pond out of

operation for a period of time before removing the

sludge. Because most of the sludge characteristics

measured (TS, VS/FS, helminth eggs, indicator organ-

isms) varied significantly both horizontally and verti-

cally in the sludge layer of the facultative ponds, and

vertically in the anaerobic pond, future efforts to

characterize WSP sludge should include the collection

of core samples from representative locations through-

out the pond.The specific conclusions from this research

include:

1. Given the range in per capita sludge accumulation

rates from 0.021 to 0.036 m3/person/yr measured in

this research, a value of 0.04m3/person/yr is a

reasonable estimate of the rate of sludge accumula-

tion in both anaerobic and facultative ponds in the

central region of Mexico.

2. Although the per capita sludge accumulation rates

were similar in the facultative and anaerobic ponds,

the distribution of the sludge was dramatically

different. In the anaerobic pond with multiple inlets

and shorter HRT the sludge distribution was uniform

throughout the pond, whereas in the three facultative

ponds with single inlets and longer HRTs, most of

the sludge accumulated directly in front of the inlet.

3. The two main processes that affect the volume of the

sludge after it is deposited—compression and anae-

robic degradation—were similar in all three ponds. A

regression equation relating the total solids concen-

tration to the thickness of the sludge layer was

developed that can be used to evaluate different

processes for sludge removal. The rate of anaerobic

degradation decreased significantly after the first

year, after which the long-term, first-order inactiva-

tion rate constant ranged from 0.042 to 0.122 yr�1 in

the different ponds.

4. Using two independent methods (sludge cores and

batch experiment), the inactivation rates of four

indicator organisms were estimated. The results


provide strong evidence that most bacterial patho-

gens are inactivated within several months in the

sludge layer, whereas the inactivation of viral

pathogens may take several years, depending on the

initial concentrations; the inactivation of Ascaris eggs

was even slower. Reasonable estimates of the

inactivation of fecal coliform bacteria, F+coliphage,

and Ascaris eggs in WSP sludge in central Mexico

can be made using first-order rate constants of 0.1,

0.01, and 0.001 d�1, respectively. The rates are

expected to be dependent on temperature, among

other factors, and may vary significantly outside this

region. In terms of the average concentrations of

helminth eggs and fecal coliform bacteria, the sludge

in the Mexicaltzingo and Texcoco ponds complied

with the Mexican standards for Class B biosolids, but

the average concentration of viable helminth eggs in

the Xalostoc sludge exceeded the value allowed for

reuse or disposal of biosolids.

Acknowledgements

The authors thank the Engineering Institute at the

National Autonomous University of Mexico, Mexico

City, for providing laboratory facilities, office space, and

institutional support for this research. In addition, we

thank Bill Sluis for designing and building the sludge

corer, Eric Van Dusen, Leopoldo Sanabria Olmos, and

Peter Nelson for their cheerful endurance collecting field

samples, German Salgado Vel!asquez, Catalina Maya

Rend !on, Elly Natty S!anchez Rodr!ıguez, and Adrianna

Romero Rosales for their dedicated laboratory help, and

Mark Sobsey for donating the coliphage host strains.

Financial support from the Fulbright Foundation and

the University of California Institute for Mexico and the

United States (UC MEXUS) was invaluable.

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