biosolid treatment

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HIGH-RATE, LINED, BIOSOLIDS LAND TREATMENT SYSTEM

PROVEN AT SACRAMENTO

Perry Schafer, P.E., Brown and Caldwell*

Kent Craney, P.E., Sacramento Regional County Sanitation District

Maria Cablao, P.E., Sacramento Regional County Sanitation DistrictSteve Wilson, CPSS, Brown and Caldwell

*2701 Prospect Park Drive, Rancho Cordova, California 95670

ABSTRACT

The Sacramento Regional County Sanitation District (California) and Brown and Caldwell have

developed a unique land treatment system for final disposition of the Districts lagooned

biosolids. The system has been pilot tested for 5 years in a 0.4 hectare (1.0 acre) synthetic-lined

land application facility. Long-term loading rates of 390 to 450 dry metric tonnes per hectare per

year (175 to 200 dry tons per acre per year) have proven acceptable. Since this is far aboveagronomic loading rates, there are no crops grown at this site. The primary purpose of the

system is cost-effective final treatment and disposition of digested, lagooned biosolids.

Since there was no known experience with using a synthetic geomembrane liner and a leachate

(infiltrate) collection system for a biosolids land application site, the District proceeded to buildand operate the pilot facility and provide long-term operation and performance testing to define

acceptable loading rates, confirm costs, and insure reliable performance of system components.

This extensive testing work has been completed and the results are summarized here. TheDistrict is proceeding on a course to implement full-scale lined land treatment and disposal sites

in 2002 and 2003.

KEY WORDS

Biosolids, land treatment, synthetic liner, leachate, infiltrate, disposal, land application.

PROJECT BACKGROUND

The Sacramento Regional County Sanitation District (District) operates the Sacramento Regional

Wastewater Treatment Plant (SRWTP), serving a population of 1.2 million people and many

local industries. The SRWTP provides secondary wastewater treatment and produces bothprimary sludge and waste activated sludge. The waste activated sludge is from a high-purity

oxygen activated sludge system. The sludges and scum are anaerobically digested and the

digested biosolids are pumped to facultative lagoons, called Solids Storage Basins (SSBs), forlong-term stabilization and storage. The lagooned biosolids are then dredged (i.e., harvested

from the lagoons), and pumped during the warm, non-rainy months (May to October, generally)

to five Dedicated Land Disposal (DLD) sites where the biosolids slurry is injected into the soils.

The total injectable area at the five DLD sites is 75 hectares (185 acres - i.e., 5 sites eachcontaining 15 hectares). Figure 1 shows a schematic diagram of this solids processing system for

the SRWTP.


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Biosolids harvested from SSBs are applied at well-above agronomic rates to the DLDs. The soilacts as a treatment system to further stabilize the biosolids material and provide final disposition

for the nonbiodegradable fraction of the solids that become part of the DLD soil matrix.

Application rates have been about 225 metric tonnes per hectare per year (100 dry tons per acre

per year) over the 20-year operation of the site.

Despite a natural thick clay layer beneath the DLDs and low soil permeabilities, the existing

DLD sites are in a state of non-compliance with State and Federal regulations due to nitrate andsalt migration to groundwater below the DLDs. The California Central Valley Regional Water

Quality Control Board (RWQCB) has directed the District to remove the DLDs from service or

modify them as appropriate. The deadline to remove the existing DLDs from service isNovember, 2001.

Figure 1 SRWTP Solids Processing Schematic Diagram Current System


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LINED DEDICATED LAND DISPOSAL CONCEPT

The District presented information to the RWQCB in 1994 indicating that providing an

impervious synthetic lining system might be a viable DLD modification alternative. Options

were evaluated and the overall economics of this approach appeared to match potential costs for

large-scale mechanical dewatering and off-site land application programs, or other beneficial usealternatives that might be available to the District. Therefore, due to potential favorable

economics, the District chose to construct a 1-acre pilot lined DLD in 1995 to test the concept.

PILOT LINED DLD CONSTRUCTION

The following list identifies the objectives of the Lined DLD (L-DLD) pilot test facility:

Evaluate maximum practical loading rates for both dredge-harvested biosolids (harvestedsolids (HS)) slurry at 4 to 8 percent total solids and dewatered biosolids cake (HS

dewatered cake) at approximately 20 percent total solids;

Evaluate soil drainage characteristics of the test cells in winter wet weather conditions,spring drying out conditions, and summer operating conditions;

Evaluate the limitations associated with biosolids application and incorporationequipment operating on L-DLDs;

Assess odor potential for high biosolids application rates; Measure infiltrate generation rates and chemical characteristics of infiltrate; and Assess potential for plugging or biofouling of geotextile filters and/or other components

of the infiltrate collection system.

A total of eight hydraulically isolated test cells were constructed in the northwest corner ofexisting DLD 1 in late 1995 and early 1996. The 8 test cells encompass 1 surface acre (0.4

hectare). The test cells were designed to model as closely as practical the systems that could beexpected in a full-scale L-DLD. Each test cell consists of a 40-mil HDPE liner, drainage layer,

filter system, and soil backfill. The soil backfill was designed to be constructed as three layers

with distinct functions. The upper 18 inches were to function as the zone of biosolids

incorporation and primary treatment. The 12 inches immediately underlying this zone weredesigned as an engineered plowplan to promote evaporation and degradation in the zone of

incorporation. The deepest zone, varying in thickness, was designed as a structural support zone.

Infiltrate collected in the drainage layer above the liner was discharged to the SRWTPheadworks. Figure 2 shows a layout of the test cells and Figure 3 shows sections through the

two types of cells. Four test cells contained a 1-foot drainrock layer, and four test cells contained

a much thinner geonet drainage layer. Major unknowns were the quantity of infiltrate liquid,characteristics of the liquid, and potential clogging of the drainage layer materials.


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Figure 2 Lined DLD Pilot Test Cell Plan


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Figure 3 Sections for Lined DLD Test Cells


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PILOT LINED DLD OPERATIONAL RESULTS

The following sections provide information, data, and results from the operation of the pilot

Lined DLD (L-DLD) test facility. The pilot facility has been operated for 5 years (1996 through

2000) and is continuing to be operated in year 2001, however, a substantially greater amount of

data were collected in the years 1996 and 1997 so that the District could make decisions onwhether to proceed to larger scale for this concept. The test site is located in the corner of DLD

1 and there is nothing visible at grade level to distinguish the 1-acre pilot lined section from the

other 36 acres of unlined DLD No. 1. The biosolids slurry injection equipment and otherdiscing and related mobile equipment moves directly over the test facility from the unlined to the

lined portion for the operations described here.

Biosolids Application To Pilot Lined DLD

Standard procedures for application and incorporation of biosolids were implemented based on

past application methods on site. These methods were modified based on the type of biosolids

and conditions encountered in the field. During the first year (1996), HS cake was applied tofour test cells at a rate of approximately 400 dry tons per acre per year (dt/ac/yr). The other four

test cells received HS slurry at a rate of approximately 166 dt/ac/yr. During the second year(1997), dewatered cake was again applied at a rate of 400 dt/ac/yr to the first four cells; whereas

the other four test cells received a combination of HS slurry and dewatered cake at a total

loading rate of approximately 400 dt/ac/yr. During 1998, 1999, and 2000, only HS slurry hasbeen applied to the pilot L-DLD facility. Table 1 presents actual quantities and loading rates.

Table 1 - Biosolids Application to Pilot L-DLD (1996 through 2000)

YearaBiosolids

TypebApplications

(#/yr)

Total DryTons Applied

(per acre)

Average DryTons per

Application(per acre)

AveragePercent Total

Solids(%)

AveragePercent Volatile

Solids(%)

1996

1996

19971997

1998

19992000

HS slurry

HS cake

HS/cakeCake

HS slurry

HS slurryHS slurry

24

24

38 (19/19)22

23

2822

166.4

409.6

375.8408.9

152.4

196142.6

6.9

17.1

9.918.6

6.6

76.5

5.7

20.9

4.2/19.219.3

5.1

54.3

52.1

52

52.3/53.954.8

51.8

5251.5

a The application season is normally early May through October each year.b Note different materials applied:

HS slurry is 4-8% solids content dredged biosolids.

HS cake is HS slurry that was dewatered on belt presses.

HS/cake is combination of slurry and cake material in a 20% HS to 80% cake (dry weight) ratio.Cake is dewatered cake that was a combination of HS and direct anaerobically digested biosolids.

Site Meteorology

Site meteorological data are not presented here. The high evaporation experienced at this site is

only achieved because of the climate that exists in the Sacramento and Central Valley area ofCalifornia in the warm, dry months of the year. Only small amounts of rain occurred in these


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test years during May (although an unusual 2.1 inches occurred in May 1998), and very small

quantities (often zero) precipitation occurred in June, July, August, and September. In October,rain averaged somewhat less than 1.0 inch per month. Most of the average annual 18 inches of

precipitation in Sacramento occurs from November through April.

Pan evaporation rates in the Sacramento area average about 11 inches per month in May, and riseto about 12 inches per month in June, July, and August. By October, pan evaporation drops to

about 6 inches per month, and in November is usually about 2 inches per month. Therefore,

application rates are usually terminated by November, even in relatively dry years.Temperatures in the summer in Sacramento often reach 35 degrees C (95 degrees F) in the

afternoon and humidity is low. Winds are significant (averaging 5 to 13 km/hour 3 to 8 miles

per hour) which help dry the biosolids-amended soils.

Soils Data

The biosolids are applied in the top 6 to 8 inches (15 to 20 cm) of soil and incorporated through

discing and plowing to about 12 to 15 inches of depth this is considered the primaryapplication zone, and, also the primary treatment zone. Through frequent discing/plowing, these

soils are quite permeable and well mixed. Beneath this zone, a more impermeable layer forms(plow pan), which limits moisture movement downward. The vast majority of applied water is

lost to evaporation. The vast majority of precipitation in the winter either runs off as surface

water (returned to SRWTP for treatment), or is retained within the soils for evaporation in thespring.

Figures 4 and 5 show soil moisture over the 1996 to 1998 period of the pilot facility. Thesurface soils (0.5 feet below ground surface) react to the precipitation of winter and somewhat to

the applied water of the biosolids, but below that level, soil moisture is relatively constant. Soilmoisture content at 2.5 feet below ground level remained relatively unchanged over time at 25

percent or less through this test period.

Figure 6 shows volatile solids content of the pilot facility surface soils (top application zone)over the 1996 to 1998 time frame. The very high application rates (up to about 400 dry tons per

acre per season especially for test cells 1 to 4), cause the increased volatile solids content of these

soils. The background volatile solids content of the general DLD soils was measured at 12 to 16percent, indicating that all DLD surface soils have increased significantly in volatile content over

the 20 year history of biosolids application. The volatile content of surface soils in the area

which have not received such biosolids amendments is more in the range of 2 to 4 percent. Thepilot facility numbers increased to the 20 percent and higher range due to the very high

application rates of this testing program.


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Figure 4 Average Soil Moisture Content Over Time Test Cells 1 through 4

Figure 5 Average Soil Moisture Content Over Time Test Cells 5 through 8


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Figure 6 Average Volatile Solids in Surface Soils

Infiltrate Monitoring Data

Table 2 presents infiltrate water monitoring information. The very low volume of infiltrate

liquid collected was of concern early in the pilot testing program. The values are on the order of

3800 liters per year (1000 gallons per year) for all 8 cells, covering 1 acre. Following

examination of the soil makeup within the lined DLD cells, it was discovered that constructionwork had compacted the soils much more than planned from the drainage layer up to the primary

treatment zone. Low permeabilities were measured in these soils which prevented anysignificant movement of moisture down to the drainage layer. Therefore, these infiltrate

quantities need to be cautiously evaluated for other situations with higher permeabilities in the

soils.

Table 2 - Infiltrate Water Quantity and Quality Data (annual averages for all pilot cells)

Parameter 1996 1997 1998 1999 2000 Averages

pH

Volume (gal/acre/year)

Conductivity (micromhos/cm)BOD (mg/l)

COD (mg/l)Nitrate (mg/l) NO3 as N

Ammonia-Nitrogen (mg/l)

6.7

1240

2810ND**

NT**2550

ND**

6.7

884

3050ND**

1751264

ND**

6.8

310

3790ND**

1701780

ND**

6.6

n/a*

6530ND**

10202230

ND**

6.6

n/a*

7995ND**

305700

ND**

6.7

811

4835ND**

417.51705

ND**

The characteristics of the infiltrate liquid show that it has near neutral pH, but relatively highdissolved solids and significant nitrate concentration. The ammonia has all been nitrified to

nitrate, and other work has shown that the majority of applied nitrogen is lost to denitrification


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and ammonia release. The nitrate concentration in infiltrate liquid (1000 to 2500 mg/l as NO3-

N) is significant, but this liquid is collected and returned to the treatment plant. The nitrogenload from this recycle is extremely low compared to plant influent nitrogen load.

The organic content within the infiltrate liquid shows that several hundred mg/l of COD exists,

but this is essentially low-biodegradable material since the BOD values were essentially non-detectable or possibly a few mg/l.

Table 3 shows metals concentrations for 3 periods of sampling (1998, 2000, and 2001). Theseare average values for the 8 test cells, but represent only 1 sampling period each year. These are

very small concentrations of metals. The concern for metals in the infiltrate is that this water

will be recycled to the SRWTP, and some small portion could become part of the plant effluentto the Sacramento River. Further evaluation of this issue shows that extraordinary conditions

would need to occur for metals concentrations in the infiltrate to reach values of concern.

Table 3 - Infiltrate Metals Testing Data (annual averages for all pilot test cells)

Year

Arsenic(micro-g/l)

Cadmium(micro-g/l)

Chromium(micro-g/l)

Copper(micro-g/l)

Lead(micro-g/l)

Mercury(micro-g/l)

Nickel(micro-g/l)

Zinc(micro-g/l)

1998

2000

2001

7

16.6

1.8

0.4

1.1

0.2

10.8

8

10.7

43.4

93.4

95.3

0.4

2

0.9

0.024

0.028

0.035

150

228

59

61

365

41

Odor Monitoring

Odor data were collected from the pilot test cells through the use of standard flux chamber

technology (based on EPA guidelines for flux chambers for gas sample collection). Gassamples were collected after biosolids application under a variety of conditions, as well as

samples following discing/plowing, and samples prior to any biosolids application. Sampleswere collected in Tedlar bags through bag-in-drum technique to avoid contamination of the gas

samples. Bagged samples were evaluated through an on-site odor panel and ASTM E-679 forcechoice triangle methodology using an IITRI dynamic dilution olfactometer.

Data for HS slurry application and normal discing and plowing showed that odor emission rateswere typically low less than 2 to 3 odor units per square foot per minute. These data occurred

despite the extremely high application rates for HS slurry. This is attributed to the very well

stabilized nature of the HS material having been subjected to long-term lagoon stabilization.Samples were taken on nearby agricultural land for moist rye grass, etc., which showed similar

values less than 1 odor unit per square foot per minute. Downwind odor levels in these cases

were non-detectable, confirming the monitoring data.

For dewatered HS cake applications, somewhat higher odor emission rates were measured values in the range of 5 to 10 odor units per square foot per minute. The higher values were

measured immediately after application of dewatered HS cake, with cake material still on the

ground surface. Following discing and incorporation of this material, somewhat lower values

were recorded. However, odor emission data were always higher for HS cake than for HS slurryapplication.


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Use of anaerobically digested biosolids (direct from digesters) as cake or slurryapplication is not recommended due to the high potential for odor problems.

If the District elects to use only HS slurry (or primarily slurry with occasionalapplications of HS cake), then the estimated maximum practical sustained loading rate is

175 dt/ac/yr.

Tillage operations should include discing to incorporate HS cake and plowing if HSslurry is used. Plowing tends to enhance soil evaporation, and should be used as needed

to reduce soil moisture in early spring, late fall, or routinely if HS slurry is used alone.

If HS cake is to be used, the District will need to purchase manure spreaders, the quantitydepending on the size and number of L-DLDs and planned operating scenarios.

From an operational standpoint, there were no differences in performance or function ofthe test cells based on the depth of the treatment zone soil. Total soil depth will have animpact on capital costs, and therefore, limited soil thickness is recommended. About 3.5

to 4 feet is believed to be the minimum allowable depth to the drainage layer due to deep

soil ripping which might be conducted from time to time.

Despite two years of above-normal precipitation (1997/1998), the as-built soilcompaction characteristics apparently limited generation of infiltrate within the test cells.Testing of the geotextile filter indicated little or no evidence of biological clogging.

Although some filter clogging due to fines or biological activity is to be expected, itappears that wide-spread, significant clogging of a geotextile filter would not occur, and a

geotextile should function adequately in this role.

Due to the general lack of infiltrate during the two-year pilot test, there was no evidenceof clogging in either the geonet or the drain rock underdrain systems. Selection of geonet

is somewhat riskier in this regard because the potential for significant loss of flow

capacity is dependent on the total void space, and the minimum constructable gravel layerthickness of 12 inches would provide more interstitial space.

Based on observed operational conditions, measured infiltrate volumes, and soil moisturedata, the relatively dense as-built soil conditions appear to be beneficial. Concern

regarding long-term accumulation of salts may lead to alternate conclusions, including

occasional deep tillage to relieve compaction, and help to wash salts from the soil system.

DISTRICT BIOSOLIDS PROGRAM

The Districts Biosolids Program is currently considering the option of constructing lined DLDsfor up to four of the existing 37-acre DLD sites to provide on-site, reliable treatment and disposal

of lagooned biosolids. In conjunction with this Lined DLD project, the District is developing a

project to privately design, build, operate and own (DBOO) a Biosolids Recycling Facility(BRF). Proposals for the BRF are expected to be received by the District in August 2001.

Additionally, the District Board of Directors has agreed to integrating biosolids recycling into its

program with a 5 to 10 percent increase in costs for recycling over the costs for Lined DLDbiosolids management. This allowance should help to maintain a recycling ethic for the

District at reasonable cost.

The District expects that a combination of on-site biosolids treatment and disposal in lined DLDsand recycling at a BRF will provide cost-effective, diversified, and reliable biosolids

management for many years. Both projects are expected to provide a minimum of 15 to 20 years


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of biosolids management for the District. Preliminary analysis of the costs associated with each

project show that there are two likely combinations of L-DLDs and BRF that could be viable:

Option # 1 - 3 lined DLD units (equivalent capacity of 60 to 70 dry tons per day)- Biosolids Recycling Facility (capacity of 20 dry tons per day at buildout)

Option # 2 - 2 lined DLD units (equivalent capacity of 40 to 45 dry tons per day)- Biosolids Recycling Facility (capacity of 45 dry tons per day at buildout)

Option # 1 would allow the District to build Lined DLDs to meet current biosolids handling

needs, while allowing future increases in biosolids production to be addressed by the BRF.

Option # 2 would require that existing biosolids processing needs be accommodated by bothLined DLDs and BRF operation, with all future increased biosolids handling needs met by the

BRF. A final decision on the direction of the Districts Biosolids Program is expected by late

2001.

The actual construction of the Lined DLD units is likely to take place within 2 constructioncontracts. Either 1 or 2 Lined DLD units will likely be constructed during the 2002 construction

season (Contract A), and either 1 or 2 Lined DLD units will likely be constructed during the2003 construction season (Contract B). Construction of the lined DLDs involves a large quantity

of earthwork and synthetic liner installation, both of which are heavily weather dependent. To

avoid construction during inclement weather, it was decided that two lined DLDs (maximum)could be reasonably built during one construction season.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the efforts of many District staff and consultants who haveparticipated in the development of the Lined DLD program and the implementation and

evaluation of the Pilot Lined DLD facility. At the District, this includes Wendell Kido, Stan

Dean, Craig Lekven, Ruben Robles, Bob Sembach, Randy Price, Mike Donahue, Lucy Boehm,

Andrew Frankel, Jorge Barajas, and Rick Johnson. Dames & Moore participated significantly inthe Pilot Lined DLD program - John Fawcett, Jeff Bold, Anne Olson. Also, John Gilmour of the

University of Arkansas, and Brown and Caldwell staff Bob Witzgall and Tracy Stigers.

REFERENCES

Brown and Caldwell, 1995. Contract Documents for construction of Pilot Test Cells CountyContract No. 3020, Sacramento Regional Wastewater Treatment Plant.

Brown and Caldwell, 2000. Design Confirmation Report Lined Dedicated Land DisposalProject, for the Sacramento Regional County Sanitation District.

Dames & Moore, 1998. Final Pilot Test Report Lined Biosolids Land Disposal Unit, SRWTP,

for the Sacramento Regional County Sanitation District.


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Gilmour, John, 1998. Sacramento Biosolids Study Decomposition and Respirometry.

February 1998.

Sacramento Regional County Sanitation District, 2001. Operational, performance, and

monitoring data on the Pilot Lined DLD over the period 1996 to 2001.