material fluxes in constructed wetlands treating septage

1

Material fluxes in constructed wetlands treating septage and their polishing systems 1 T. Koottatep*, N. Surinkul*, A.S.M. Kamal*, C. Polprasert*, A. Montangero**, K. Doulaye**, M. Strauss**

* School of Environment, Resources and Development (SERD), Asian Institute of Technology (AIT), P.O. Box 4 Klong Laung, Pathumthani 12120, Thailand. [email protected] ** Department for Water and Sanitation in Developing Countries (SANDEC), Swiss Federal Institute for Environmental Science and Technology (EAWAG) , P.O. Box 611, CH-8600 Duebendorf, Switzerland.

Abstract Vertical-flow constructed wetlands (CWs), constitute a promising system for septage treatment offering satisfactory treatment performance at much lower construction and operating cost than conventional sludge dewatering processes. This study discusses the possible pathways of material fluxes in the pilot-scale constructed wetland (CW) units for septage treatment at AIT, which have been operated since 1997. The experimental results of three CW units planted with narrow-leave cattails (Typha augustifolia), operating in a vertical-flow mode at a solids loading rate of 250 kg total solid (TS)/m2.yr and 6-day intermittent percolate impoundment, are presented. Material fluxes in the CW units treating septage include (1) accumulated solids on the bed surface; (2) plant yield and harvesting; (3) water evaporation or volatilization of the gaseous constituents; and (4) the liquid fraction in the CW percolate. A sludge layer of about 80 cm has been retained on the media surface after operating the CW units for 7 years without removal of the dewatered septage. The retained solids exhibited TVS values of 60 – 70%, helminth egg concentrations of 60 – 70 eggs/g of TS, nutritional values comparable to organic fertilizers, while having low heavy metal contents. Cattail harvesting proved advantageous in order to maintain high plant yields. Nutrient and heavy metal uptake by the plants was low.. Cattails reduce the dewatering time due to evapotranspiration processes. Dewatered solids TS contents of20 – 30% TS were attained within one week after feeding.. Although removal efficiencies of greater than 90% were achieved in the percolating liquid, contaminant levels in the percolate still exceeded discharge standards. Therefore, the percolate of CWs treating septage has to be subjected to polishing treatment in order to meet the discharge limits., Attached-growth waste stabilization ponds (AGWSP) have been experimented upon to determine an appropriate percolate polishing system. . AGWSP with plastic media achieved COD and TKN removals of 85% but were unable to meet the stringent domestic effluent standards of Thailand. The treated percolate, while being too saline for direct agricultural use, may be diluted with natural water or treated wastewater if feasible to allow its use in agriculture.

Keywords Material fluxes, percolate polishing systems, septage treatment; vertical-flow constructed wetlands

INTRODUCTION Sludge or septage treatment in CW systems depends mainly on various treatment mechanisms including solids accumulation and mineralization, biodegradation, chemical precipitation and adsorption, nitrification/denitrification and plant uptake. Wittgren and Tobiason (1995) and Koottatep and Polprasert (1997) reported that plant uptake of nitrogen (N) might play an important 1 In: Proceedings, 9th International IWA Specialist Group Conference on Wetlands Systems for Water Pollution

Control and 6th International IWA Specialist Group Conference on Waste Stabilization Ponds, Avignon, France, 27 Sept. – 1 Oct., 2004.

2

role in removing N contents in CW units treating wastewater, based on mass balance analyses. In addition to expressing the relevant treatment mechanisms, mass balance analyses can depict the residual pollution loads from CW units to be discharged into the receiving environment. The residual pollution may include the accumulated septage (so called biosolids) in CW units and the percolate, which typically require an appropriate conditioning prior to discharge or reuse. Even though the CW units for septage treatment could achieve relatively high removal efficiencies (Koottatep et al., 2004); the pollutant concentrations in the CW percolate remained somewhat higher than the effluent standards. Furthermore, the CW biosolids may contain pathogen concentrations including helminth eggs, which might preclude agricultural use without further storage and/or dewatering. This paper describes the material fluxes in the CW units treating septage for total solids, water, and nitrogen and discusses possible removal mechanisms. Moreover, potential solutions for percolate treatment and the reuse of biosolids are discussed in the paper.

METHODS

Setup of experimental units CW units - Three pilot-scale CW units, each with a surface area of 5x5 m and a 65-cm substrata layer, were established at the Environmental Research Station of AIT and operated in a vertical-flow mode. The substrata in CW units comprise a 10-cm layer of fine sand, a 15-cm layer of small gravel, and 40-cm layer of large gravel from top to bottom, while a free board of 1 m was allowed for accumulation of dewatered septage. Each CW unit was planted with narrow-leave cattails at an initial density of 10-15 shoots/m2. The drainage system of CW unit includes hollow concrete blocks, each with dimensions of 20 x 40 x 16 cm (width x length x hollow space), and perforated PVC pipes with a diameter of 20-cm. Mounted on the drainage system are ventilation pipes of the same diameter and extending approximately 1 m over the top edge of the units (Fig. 1). Operation conditions of these CW units treating septage are described in Koottatep et al., 2004.

Figure 1. Schematic diagrams of pilot-scale CW units and its percolate polishing systems Percolate polishing systems – In this study, the percolate polishing system included four pilot-scale attached-growth waste stabilization pond (AGWSP) units, each with dimensions of 1.0 x 4.0 x 1.0 m. To investigate and compare the treatment performance with the increased biofilm biomass in the AGWSP systems, a conventional waste stabilization pond with no media has been operated. Three types of attached-growth media were used in parallel-operated units, viz. a unit with Globeflux G-70 plastic media at 5% of pond water volume, one with Manila rope at a diameter of 6-mm with a total length of 100 m wrapped with 2-mm nylon string, and one with 5-cm rolls (?) of low-cost shading plastic sheet with a total length of 100 m (Koottatep et al., 2000). In addition, the experiments were expanded to investigate the operational performances of double AGWSP in

BARBARSCREENSCREEN

STORAGESTORAGETANKTANK

MIXINGMIXINGTANKTANK

SEPTAGESEPTAGEPUMPPUMP

ROCKROCKFILTERFILTER AGWSPAGWSP

ININ--SERIESSERIES

CONSTRUCTEDCONSTRUCTEDWETLAND NO. 1WETLAND NO. 1



POLISHINGPOLISHINGPONDSPONDS

EFFLUENT EFFLUENT TO CANALTO CANAL

BARBARSCREENSCREEN

STORAGESTORAGETANKTANK

MIXINGMIXINGTANKTANK

SEPTAGESEPTAGEPUMPPUMP

ROCKROCKFILTERFILTER AGWSPAGWSP

ININ--SERIESSERIES




POLISHINGPOLISHINGPONDSPONDS

EFFLUENT EFFLUENT TO CANALTO CANAL

VVERTICALERTICAL--FFLOW LOW CCONSTRUCTED ONSTRUCTED WWETLANDSETLANDS

RAW RAW SEPTAGESEPTAGE

WOODWOODPILEPILE

FERROFERRO--CEMENTCEMENTWALLWALL

REINFORCEDREINFORCEDCONCRETE SLABCONCRETE SLAB

HOLLOWHOLLOWCONCRETECONCRETE

BLOCKBLOCK

VENTILATION VENTILATION PIPEPIPE

DRAINAGEDRAINAGEPIPEPIPE

PERCOLATE PERCOLATE TANK TANK

CATTAILSCATTAILS

PERCOLATE PERCOLATE TO TO AGWSPAGWSP

IN SERIESIN SERIES

VVERTICALERTICAL--FFLOW LOW CCONSTRUCTED ONSTRUCTED WWETLANDSETLANDS

RAW RAW SEPTAGESEPTAGE

WOODWOODPILEPILE

FERROFERRO--CEMENTCEMENTWALLWALL

REINFORCEDREINFORCEDCONCRETE SLABCONCRETE SLAB

HOLLOWHOLLOWCONCRETECONCRETE

BLOCKBLOCK

VENTILATION VENTILATION PIPEPIPE

DRAINAGEDRAINAGEPIPEPIPE

PERCOLATE PERCOLATE TANK TANK

CATTAILSCATTAILS

PERCOLATE PERCOLATE TO TO AGWSPAGWSP

IN SERIESIN SERIES

3

combination with the horizontal flow rock filter (HRF) and the polishing pond prior to discharging into a canal, as illustrated in Fig. 2. Another experiment on the polishing of CW percolate was the application of CW unit no. 3 planted with Canna species (an ornamental emergent plant) operated in series with CW unit 1.

4

Figure 2: Functional sketches of unit operations for polishing percolate

RESULTS AND DISCUSSION

Mass balances in CW units Mass balances of water, solid and nitrogen across the CW beds treating septage using the data of one year of bed operations are depicted in Fig. 3. It can be seen that half of the water fed with septage was evapotranspirated and 45% was drained, while the rest, 5%, was retained in the accumulated solids. The TS mass retained on the CW bed accounted for 28%, while TS in the

Percolate feeding tank

AGWSP-1AGWSP-2

Polishing pond

* ERT = Effluent receiving tank** HRF = Horizontal-flow rock filter

AGWSP-3

Mixer-equippedfeeding tankScreening

tank

Storage tank

BMA

CW-1

CW-2

CW-3

ERT*-1

ERT*-2

ERT*-3

To constructedwetland units

From screening & storage units

To AGWSP units& polishing ponds

AGWSP-4


HRF**

From constructedwetland units

To canal


AGWSP-1AGWSP-2

Polishing pond

* ERT = Effluent receiving tank** HRF = Horizontal-flow rock filter

AGWSP-3

Mixer-equippedfeeding tankMixer-equippedfeeding tankScreening

tankScreening

tank

Storage tank

BMABMA

CW-1

CW-2

CW-3

ERT*-1

ERT*-2

ERT*-3

To constructedwetland units

From screening & storage units

To AGWSP units& polishing ponds

AGWSP-4


HRF**

From constructedwetland units

To canal

5

percolate was 6%. It can be inferred that the rest (66%) of the TS mass constituted the unaccounted-for balance, which had undergone biochemical reactions and solids accumulation in the wetland substrata. In addition, the TS concentrations of 16 - 32% observed in accumulated septage on the CW beds were comparable to those of other dewatering processes such as through centrifuging (23%), belt pressing (24%), and filter pressing (32%).. The N mass in the accumulated septage and CW percolate accounted for only 13% and 5%, respectively, of the total N loaded through septage. Losses of N from CW units of about 82% could be due to ammonia volatilization, denitrification, microbial and plant uptake, and N accumulation in the CW substrata.

Figure 3. Water, TS and N balance in CW units treating septage

Characteristics of biosolids In order to observe the changes of characteristics in accumulated septage, the operation of CW unit 2 was stopped after 4 years of operation in February 2001. According to the biosolids characteristics as shown in Table 1, the total volatile solid (TVS) have reduced substantially from 64% in Sep 2000 to 41% in Feb 2003, likely due to the mineralization of organic contents in the accumulated biosolids. On the contrary, the TS concentration in biosolid was increased from 29% to 47%.

Table 1. Biosolids characteristics of CW unit 2 Period a-N

(mg/kg) a-P

(mg/kg) a-K

(mg/kg) TS (%)

TVS (%)

September 2000 1,000 3,400 135 29 64 August 2001 - - - 36 55 May 2002 - - - 57 47 February 2003 1,060 3,922 152 47 41

Note: Loading of raw septage in CW unit 2 was stopped in February, 2001 not in June or August 2000 already ?. No significant change in the characteristics of biosolids in terms of available nutrients, viz. a-N, a-P and a-K concentrations has been observed after stopping the operation. It was also found that the available N, P, and K accounted for about 5 – 8% of the total N, P and K contained in the accumulated solids (Table 2), which together with pathogens and heavy metals are an important criteria for determining the suitability of biosolids for agricultural applications . Biosolids samples

39%

Perc

olat

e (1

1%)

TS Balance

Min

eral

izat

ion,

bio

degr

adat

ion

and

accu

mul

atio

n in

filte

r med

ia

FS (100%) Accumulated septage (5%)

Evap

otra

nspi

ratio

n (5

0%)

Perc

olat

e (4

5%)

Water Balance

NH

3 vol

atili

zatio

n,

deni

trific

atio

n an

d pl

ant

upta

ke (

82%

) N Balance

FS (100%)

Perc

olat

e (5

%)

Accumulated septage (13%)

Accumulated septage (50%)

FS (100%)

6

were collected from the upper (15 cm from surface) and deeper (45 cm from surface) layers of the CW units. Zn exhibited the highest concentration among heavy metals analysed for in the raw septage of Bangkok (Koottatep et al.,2004) , probably because it has been used as an ingredient in toilet cleansing agents. The available N and P in the deeper was converted into available and free nitrogen forms likely because CW 2 was at rest for two years prior to analysis. This could result in the lowest TN content (2.58%) in a deeper layer of CW unit 2. However, the available N, P and K contents in the biosolids after stopping operation for 2 years are still at high concentrations, probably due to the biodegradation of organic-N and organic-P and the leaching of inorganic-N and inorganic-P from the upper layer to the deeper layer of biosolids in the CW unit.

Table 2. Nutrients and heavy metals in biosolids accumulated on the constructed wetlands (after how many years of operation ?)

Units T-N (%)

a-N (mg/kg)

T-P (%)

a -P (mg/kg)

T-K (%)

a-K (mg/kg)

T-Zn (mg/kg)

CW 1 (upper)* 3.09 433 1.95 2,442 0.20 520 2,360 CW 1 (deeper)** 3.03 534 1.75 2,946 0.19 340 3,590 CW 2 (upper) 3.14 770 1.94 2,585 0.15 138 1,600 CW 2 (deeper) 2.58 1,350 2.68 5,260 0.18 165 4,410

Note: Average from 12 composite samples, ‘a’ means available and ‘T’ means total * upper = 15 cm from surface and **deeper = 45 cm from surface The Zn content in biosolids at the deeper layer ranged from 3,500 to 4,400 mg/kg, i.e. higher than those in the upper layers of CW unit. A preliminary recommendation for the limiting concentration of Zn content in sludge for agriculture reuse is 3,000 mg/kg (AIT, 1998). Mixing of biosolids with soil may be required if the deeper biosolid layer is used; otherwise it would cause detrimental effects on microorganisms living in the applied area.

Percolate polishing Attached Growth Waste Stabilization Pond (AGWSP) – Even achieving the relatively high treatment efficiencies in CW units treating septage, the CW percolate remains relatively high in COD, TKN, TS and SS, which would require a polishing system prior to discharge. Based on the pilot-scale results of AGWSP units treating the CW percolate, the AGWSP units could achieve the higher treatment efficiencies as compared with a control unit without attached-growth media at the hydraulic retention time (HRT) of 10 days (Table 3). At the relatively long HRT, the AGWSP units could not achieve the high COD removal probably because of the low biodegradability in the CW percolate as evidenced by the low BOD to COD ratio. Probably due to the high dissolved solid contents in the CW percolate, the TS removal efficiencies in the AGWSP and control units were relatively low. However, the data in Table 3 shows the beneficial effects of the attached-growth media in reduction of the SS contents. TKN and NH3 removal efficiencies of AGWSP and control units were not significantly different, but the NO3 removal efficiencies of AGWSP units were apparently higher than those obtained from a control unit. It could be due to some of the attached-growth biofilm were denitrifying bacteria, performing denitrification process in removal of NO3 contents.

Table 3. Average influent and effluent SS, COD, TKN, NH3-N, and NO3-N concentrations of AGWSP units operating at HRT of 10 days

Parameter *, mg/L Unit No.

Attached-growth media SS TS COD TKN NH3 NO3

1 No media (control) 77 (73) 1,670 (15) 183 (47) 28 (58) 16 (71) 16 (14)

7

2 Globe-flux 75 (74) 1,530 (21) 180 (48) 25 (63) 11 (80) 13 (30)3 Manila rope 51 (82) 1,690 (13) 165 (52) 28 (59) 19 (65) 12 (38)4 Plastic sheet 50 (83) 1,650 (15) 115 (67) 22 (67) 12 (78) 12 (33)

Influent 290 1,950 350 67 55 20 * Removal efficiencies are given in parentheses Considering the costs, durability and ease of maintenance of the attached growth media, it was noticed that the plastic was the suitable media for the AGWSP units. The commercial Globeflux G70 media are quite costly, whereas the manila rope is obviously not durable for wastewater treatment. Combination of polishing systems – Polishing of CW percolate was experimented not only in AGWSP units, but also in combination with other systems such as horizontal rock filtration (HRF), AGWSP in-series, and CW unit planted with Canna species. These investigations aimed to determine the appropriate polishing systems of the CW percolate. To partially remove of SS contents, the CW percolate has been treated in a HRF unit at the HRT of 30 min, prior to feeding into the AGWSP in-series. According to the experimental data obtained as shown in Table 4, the HRF unit with the HRT of 30 min could achieve the SS, TS, COD TKN, NH3, and NO3 removal efficiencies of 63, 24, 61, 60, 61 and 49%, respectively. However, after operating couple of months, the clogging in HRF units was observed, causing the operational difficulties during washing the HRF media.

Table 4. Performance of HRF treating CW percolate at HRT of 30 min

Parameter* Influent )mg/L( Effluent )mg/L( Removal (%)

SS 227 84 63 TS 2,735 2,083 24 COD 495 194 61 TKN 122 49 60 NH3 94 37 61

NO3 61 31 49 * Average from 40 samples Two sets of AGWSP in-series, each with plastic-sheet AGWSP unit, were operated at the HRT of 20 days; one set receiving HRF effluent and another receiving CW percolate. Fig. 4 shows treatment performance of AGWSP in series in terms of TS and SS concentrations. Based on the statistical analyses of experimental results, the AGWSP in-series with pre-treatment by HRF did not show significant improvement in treatment performance as compared with those of the AGWSP in-series without HRF. Similar phenomenon was observed for other parameters including COD, TKN, NH3 and NO3 concentrations, as shown in Table 5.

Table 5. Treatment performance of AGWSP in-series and Canna-planted CW Effluent concentration (mg/L)

Parameter AGWSP in-series HRF + AGWSP in-series

Canna-planted CW

TS 1,900 1,700 2,800 SS 60 40 14 COD 150 110 180 TKN 23 14 7 NH3 15 8 3 NO3 35 17 82

8

In order to determine and compare treatment performance of the percolate polishing systems, the CW unit 3 was re-planted with Canna species and experimented by treating the percolate from CW unit 1 at hydraulic loading rate (HLR) of 25 cm/d. The experimental results in Table 5 revealed that the Canna-planted CW unit could achieve the better polishing performance in terms of SS and NH3, while the HRF + AGWSP in-series showed higher COD removal efficiencies. (a) Treating CW percolate (b) Treating HRF effluent

Figure 4. Treatment performance of AGWSP in-series

It could be remarked from the abovementioned results that, in order to further treat the percolate of CW units treating septage, the polishing systems such as AGWSP, HRF, and Canna-planted CW should be installed and operated at the appropriate HRT or arranging the polishing systems in-series. However, economic appraisal of the effluent standards should be carefully discussed otherwise the costs for polishing systems would be unavoidably expensive. Alternatively, the treated effluent can be used for irrigation or other agricultural practices in order to reuse the remaining nutritional constituents.

CONCLUSIONS The material flux analyses of the CW units treating septage reveal that about 50% of water content in the septage evapotranspirated, resulting the dewatered septage with TS contents of 16 – 32% at the same magnitude of conventional dewatering processes. The major sink of TS and N contents of 30% and 13% are accumulated in the dewatered septage, whereas the remaining fluxes in the

0

500

1000

1500

2000

2500

3000

Sep-98 Nov-98 Dec-98 Feb-99 Mar-99 May-99 Jul-99 Aug-99

Operating periodCo

ncen

trat

ion,

mg/

L

Influent TS Effluent TS no. 1 Effluent TS no. 2

0

20

40

60

80

100

120

140

160


Operating period

Conc

entr

atio

n, m

g/L

Influent SS Effluent SS no. 1 Effluent SS no. 2

0

50

100

150

200

250

300

350

400

450


Operat ing period

Conc

entr

atio

n, m

g/L

Influent SS Effluent SS no. 1 Effluent SS no. 2

0

1000

2000

3000

4000

5000

6000


Operat ing period

Conc

entr

atio

n, m

g/L

Influent TS Effluent TS no. 1 Effluent TS no. 2

9

percolate are 11% and 5%, respectively. The unaccounted balance of over 60% of TS flux might undergo through the transformation by mineralization and biodegradation. The NH3 volatilization, denitrification reaction and N plant uptake in CW units could account for 82% of N balance, as evidenced by the reduction of NO3 contents in CW percolate. Characteristics of the dewatered septage or biosolid in terms of fertilizing values are found in the same magnitudes of organic waste composts, but having slightly high Zn contents especially at the deeper layer of biosolid. Among various attached-growth media in AGWSP units operating at the HRT of 10 days, the plastic sheet media is considered to be the most appropriate media with respect to their costs, durability and maintenance. The operation of AGWSP in-series at the HRT of 20 days showed the promising treatment performance in terms of TS, COD, TKN, NH3 and NO3 either with or without HRF unit. Percolate polishing by the Canna-planted CW unit at the HLR of 25 cm/day also showed the promising treatment efficiencies. As an alternative to percolate polishing, the reuse of CW percolate for agricultures should be considered due to its high nutritional constituents.

REFERENCES Koottatep, T. and Polprasert, C., (1997). Role of plant uptake on nitrogen removal in constructed

wetlands located in tropics, Wat. Sci. & Tech., Vol. 36, No. 12, pp. 1-8. Koottatep, T., Polprasert, C., and Surinkul, N., (2000) Septage Treatment in Constructed Wetlands

and Attached-Growth Waste Stabilisation Ponds, Phase 2a, Final Report, submitted to the Swiss Federal Institute for Environmental Science and Technology (EAWAG), Switzerland, 75 p.

Koottatep, T., Surinkul, N., Polprasert, C., Kamal, A.S.M., and Strauss, M. (2004). Treatment of septage in constructed wetlands in tropical climate – Lessons learnt after seven years of operation, Proc. of the 9th International Conference on Wetland Systems for Water Pollution Control, Avignon, France, 27 – 30 September.

Wittgren, H. B. and Tobiason, S. (1995). Nitrogen removal from pretreated wastewater in surface flow wetlands, Wat. Sci. & Tech., Vol. 32, No. 3, pp. 69-78.

material fluxes in constructed wetlands treating septage

Documents