A Solar Aquatic System Septage Treatment Plant BY JOHN M. TEAL AND SUSAN B. PETERSON astewater discharged from U S . homes, businesses, and industries is generally treated on site by means of a septic tank and leaching field, or it is conveyed off site through a collection system to a wastewater treatment facility. Septic tank systems are most commonly used for on-site disposal of sewage wastes in the United States, with almost onethird of housing units currently using this form of wastewater treatment ( I ) . The systems produce a byproduct called septage, the material pumped from septic tanks when they are cleaned. Wastewater treatment plants produce their own type of byproduct, known as sludge or biosolids. Septage is harder to treat than sewage sludge because the more readily degradable organic materials have decomposed during the two to five years between cleanings, intervals typical of domestic systems. T h u s , for similar waste streams, the resistant organic compounds and heavy metals concentrate at relatively higher levels in septage than in sludge from a sewage treatment plant because the sewage sludges still contain degradable organics. Historically, we have disposed of septage by cotreatment in sewage plants, limiting the septage to a fraction of the sewage inputs to control organic loading and sludge production. In addition, where open land is available, septage has been applied to agricultural fields, forests, or “wastelands.” In more densely populated areas with limited agricultural land, such as Cape Cod, dumping septage into “lagoons”
W
34 Envimn.
Sci. Technol., Vol. 27. No. 1, 1993
has been the standard disposal technique for nearly 100 years. Lagoons on Cape Cod are shallow basins dug into the porous soils of the glacial moraine or outwash plain. “Treatment” consists of pumping raw septage from pumper trucks into the lagoon, whereupon a portion of the solids settles out and
the liquids either evaporate or percolate into the ground. Eventually the solids produce an impenetrable layer on the bottom of the lagoon, and because rainfall exceeds evaporation, the lagoons eventually fill up. When they are full, the supernatant is directed to a rapid infiltration basin and the solids on the bot-
0013-936X/93/0927-34$04.00/0 0 1992 American Chemical Society
tom a r e a l l o w e d to d r y . T h e from within the town. The town has remaining materials are disposed of no sewers and decided in 1989, afin a landfill. ter considering alternatives for sevThis t e c h n i q u e p o l l u t e s t h e eral years, to support an innovative groundwater. After 50-100 years of septage treatment system on a pilot this practice, pollution plumes from basis. The taxpayers were motithese sites have become a water vated by a combination of cost savsupply problem as nitrate-enriched ings, commitment to environmental groundwater flows toward well quality, and the desire to find a betfields (2-4). Lagoons are being ter way to treat septage. phased out on Cape Cod and in many Only a few waste treatment plants other jurisdictions, but the plumes in the United States are dedicated to remain. For example, a plume of septage processing; most use modicontaminated groundwater in Fal- fied standard treatment techniques, mouth, MA, resulting from sewage, treating septage as if it were thick septage, and sludge disposal forced sewage or thin sludge. Ecological the closing of a town well and pri- Engineering Associates, a Massavate wells over an area of several chusetts-based wastewater treatsquare kilometers. The direct costs ment company, manages Harwich's related to this groundwater contam- septage by using an innovative, ination total several million dollars. biologically engineered septage Pollution from treatment system, wastewater disknown commerposal is not limited cially as a Solar to groundwater Aquatics treatSEPTIC TANK supplies, because ment system the contaminants (SAS). The system flow with t h e is based o n engronndwater, then closed, artificial enter and cause euwetlands and trophication i n aquaculture sysfreshwater ponds tems (9). and coastal waters. The Harwich niNitrate causes the lot was designed most damage in as a full-scale coastal waters heplant, capable of cause productivity treating roughly in those waters l;s half the septage limited by nitrogen generated in the supply (5-7). Local t o w n . Construcwaters around I tion lasted from Cape Cod are polNovember 1989 to luted by nitrates at an increasing rate March 1990, and operations began as more people settle on the Cape year immediately thereafter. The system round (8). was evaluated over the first six A few communities, having rec- months, with some changes in meognized the importance of nutrient chanics hut no change in process. [primarily nitrogen) control in Process changes were made after wastewater treatment, require a t h e s e c o n d s i x - m o n t h p e r i o d . treatment technology that will en- Steady-state operations began in sure low levels of nitrogen in the June 1991 and continued through discharge. Because nutrient control March 1992. was required infrequently in MassaSystem description chusetts until recently, regulators consider most of these technologies The SAS begins with standard to be innovative, requiring consid- headworks of a receiving station erable pilot plant operation before and degritter. This is followed by permitting permanent construction t w o aerated a n d mixed 40-m3 a n d operation. T h e system o n (10,000 gal) in-ground tanks. A which we report here operated as a blending (equalization) tank is pilot plant from 1990 to 1992 in needed to buffer variations among Harwich, MA. truckloads and to account for the Harwich is a town of approxi- daily, weekly, and monthly variamately 10,000 year-round residents tions in septage pumpings throughwith a JulylAugust peak population out the town. Treatment, including that may reach 30,000. The lagoons bacterial additions for grease deat the town's landfill were e m i t composition and nitrogen cycling, ted to receive u p to 4 7 my day-' begins in the in-ground tanks. The (12,500 gal day-') oftrucked septage downstream end of the second
SYSTEMS ARE MOST COMMONLY USED FOR ON-SIT1 DISPOSAL OF SEWAGE WASTES I N THE UNITED STATES
equalization tank is baffled and not aerated and serves as a gravity clarifier. No chemicals or polymers are needed for settling. The rest of the SAS is in a 465-mZ (5000 ft2) commercial greenhouse. There are three treatment lines, each with ten 1.5-m-high, 2.3-m3 translucent tanks, a marsh, four more tanks, and a final marsh. The first nine tanks are aerated with fine bubbles. Tank 10 is another gravity separator from which solids are pumped back to the equalization tanks. Flow from the bottom of one tank to the surface of the next is caused by gravity. The tanks are covered with floating or racked plants with roots extending 1-5 ft into the septage. Water hyacinths and willows are the dominant vegetation; there are also tubificid worms, moth flies, and snails. L i q u i d from Tank 1 0 flows through a sand filter into the gravelfilled marsh planted with Scirpus and reed canary grass. The marsh effluent is pumped into the first of the final four tanks and flows by gravity through the tanks and final marsh into a sump from which it is pumped through a UV sterilizing unit. In the second half of the system a complete ecosystem thrives, including snails, crawfish, fish, and a broad range of ornamental vegetation. To develop additional information about the role of the marsh in septage treatment, the middle and end marshes on one line (Line C) were doubled in area in September 1991. The data from the single marshes (Lines A and B] are reported separately from the double marshes data in Figure 1 [top). The water hyacinths and willows in the first nine tanks take up less than 2 % of the nutrients in the septage; their roots have an extensive surface area particularly suitable as bacterial habitat. The grasslike plants in the marsh provide bacterial habitat, organic compounds needed to maintain anoxia, and a carbon source for dentrifying bacteria. The plants in the final tanks illustrate the range of vegetation that can he grown hydroponically in treated septage. Also within the greenhouse is a sludge stabilization line of eight aerated tanks followed by two settling tanks. The stabilized sludge is sent as waste to the old lagoons in the landfill. It could be pumped to reed beds for drying and passive composting; this process has not yet been permitted in Massachusetts, although it appears attractive for Environ. Sci. Technol.. Vol. 27. NO. 1. 1993 35
small systems with low volumes of solids. Alternatively, the sludge could he dewatered on a sand drying bed and composted, or it could be put into a landfill. In either case, underdrains would collect liquids from the sludge and return them to the equalization tanks. Before the system was reconfigured in spring 1991, sludge was not separated from the flow stream until it reached Tank 10 in the greenhouse. However, this resulted in excessive ammonia generation within the system and high solids loading to the marshes.
System function The Harwich SAS septage pilot was designed to treat 4.5 m3 day-' (1200gal/day-') per line with detention times of 10 days. Detention time has been from 11 to 1 2 days during the steady-state period. Spring, summer, and fall temperatures of the liquids vary between 20 and 27 "C, and winter temperatures are as low as 10 "C. A major concern for any wastewater treatment technology is consistent yearround performance. The Solar Aquatics process needed to treat septage effectively even during the coldest months of the year. The data illustrate that temperature fluctuations in the septage did not affect system performance. An interesting aspect of working in a Solar Aquatic greenhouse is that effluent volumes do not match influent ones because of evapotranspiration from the tanks and plants within the SAS. Evapotranspirational losses average 15% hut are greatest in the summer. Additional liquid losses are accounted for bv trackine the volume of waste siudge. The septage is a highly variable mixture when it is delivered to the SAS plant in volumes ranging from 1to 10 m3. The data identified as influent are from samples taken from the first equalization tank where several truckloads were mixed. Harwich septage is less concentrated than EPA design standards (IO)but similar to that from other Massachusetts towns ( 2 2 , Table 1). The performance data are summarized in Table 2 and shown in detail for total suspended solids (TSS), fiveday biological oxygen demand (BOD,), and nitrogen species in Figure 1 (bottom]. All average properties, with or without the doubled marshes, met the anticipated discharge limits for a groundwater discharge permit of 30 mglL BOD,, 30 Y
36 Environ. Sci. Technol., VoI. 27.No. 1. 1993
.E 1
A comparison of septage constituents (in m IL) from four Massachusetts towns with EPA deslgn Stan ards'
il
EPA
CWtHumt standard BOD, TSS TN TDP
7000
15,000 700 250
"4
Yarmouth (5E)
mean
(Muns
man (SE)
Wayland mean (SE)
HaMch
mun
(SE)
2090 (154) 2650 (91) 1100 (61) 1410 (73) 4290 (378) 5180 (139) 3580 (269) 4960 (257) 325 (18) NA 233 (20) 224 (12) 366 (4) NA 48 2 (NA) 45.7 (1.8) 131 (NA) NA 110 (NA) 52.5 (2.4) . .
* BOD,, flve.@a biological 0 gen demand; TSS, total suspended solids. TN, total nitmgen- TDP low dissolved $&asphales: N X not available. Harwich dala are from the entire pilol operating p d nod.
TABLE 2
Influent and effluent values (in mglL) for SAS Harwich septage treatment plant' Constilwnt
TSS BOD, TN Organic N "4
NO,
TP
Ortho-P
' V a l m ware ana
InRwnI (SE)
5780 (990) 1740 (362) 187 (179) 153 (174) 32.7 (256) 0.9 (0.08) 43.0 (3.5) 148 (1.1)
Elfluem mean (SE)
Line c eiliwnl mean (SE)
19 8 6 74 9 62 4.54 0 34 4.74 6 52 4 96
21.0 8.6 5.98 4.44 0.31 1.17 5.98 3.32
(1 23)
(0.68) (0.44) (0.21) (0.06) (0.49) (0.32) (0.34)
(1.03) (0.62) (0.32) (0.23) (0.03) (0.15) (0.32) (0.23)
zed by a mnlhed fa0trom weekly M m Ies tahen m m g In8 steaoy.smie pema.
On 1991 rnrabghharch 1992 TSS. total sdspendsa so&% BOD,. lve.day biocnemca oxygen demand. TN. lola. n4lmgen; TP lolai phospnate: Ormo-P Onho onospnales ne C data are la ned IiOm the line wth enarged marsnes.
mg/L TSS, and 10 mg/L total nitrogen. Line C with doubled marshes significantly reduced the effluent nitrogen concentration without changing other parameters. Organic nitrogen in the effluent averaged 4.4 mg/L in Line C with the expanded marsh during the steady-state period. Most of the residual nitrogen during this period, 4.4 of the 6.0 mg/L, or 7356, is organic and apparently quite refkactory. If it is highly resistant to degradation it should have a minimal environmental effect. Previous analyses showed that the Harwich effluent contained little in the way of such potentially damaging compounds as the pesticide DDT and its breakdown products or polychlorinated hiphenols (PCBs) (9). Because the pilot system was not operated for phosphate removal, its reduction is incidental to nitrogen removal. Eighty-four percent of total influent phosphate is removed from the waste stream, mostly by phosphate sorption to sludges or by polyphosphates also removed with the sludges. Only 3% is removed by plant harvest. The decrease in phosphate that resulted from the dou-
bling of marsh size in Line C will probably be temporary. If phosphate is sorbed onto marsh particulates, the system will revert to its previous behavior with regard to phosphate when the sorption sites are saturated. The coliform count is reduced by several orders of magnitude as the waste stream moves through the system, but because the effluent receives UV treatment before discharge, we do not show the data. The discharge limit is 200 colonies of fecal E. coli per 100 mL. Net total solids degradation during the routine operations phase was about zero, indicating that the total solids removed as sludge were equal to the amount in the influent septage. The headworks grit removal system used in the pilot was not very efficient; thus, some of the residual solids in the system were grit. The regular harvest of plants from atop the tanks accounted for only slightly more than 2% of the solids. During the three-month period from November 1990 through January 1991, total nitrogen loss within the greenhouse amounted to 30% of
Total
out ~
the influent. This is the portion not found in sludge, plants, or effluent; therefore, it must have been lost to the air by denitrification. The estimate for loss to air via denitrification during the routine operation phase is 32% of influent nitrogen. Most of the denitrification occurred in the anoxic marsh sediments and in the equalization tanks; some may also have taken place in the solar tanks, presumably in anoxic microzones associated with particles or plant roots. The portion of nitrogen removed as vegetation harvest was less than 2% of the total.
Conclusions The SAS technology, using wetland and aquaculture ecosystems enclosed and concentrated within a greenhouse, is very successful in treating raw septage to permit standards (BOD,, 30 mglL; TSS, 30 mg/ L; total nitrogen, 1Omg/L; fecal
larae mar!
E. coli, 200 colonies per 100 mL). Natural systems of rivers, wetlands, ponds, and mud flats have treated organically rich waters for eons. The innovation in SAS technology involves concentrating the system, optimizing the mix of biological components, controlling the process, and treating a concentrated waste through all seasons. In June 1992, the SAS pilot reviewed by the Massachusetts Department of Environmental Protection was found to produce an effluent that consistently met Class I drinking-water standards. That designation i s needed for a commercial facility to operate in the most fragile environments.
(21 LeBlanc, D. R. ”Sewage Plume in a Sand and Gravel Aquifer, Cape Cod. MA.” USGS Water Supply Paper No. 2218. US. Government Printing Office: Washington, DC, 1984. (3) Ceazan, M. L.; Thurman, E . M.; Smith, R. L. Environ. Sci. Technol. 1988,23,140248. (4) Thurman, E. M.; Barber, L. B., Jr.; LeBlanc, D. I. Contam. Hydml. 1986, 1. 143-61. (5) Ryther, 1. H.; Dunstan, W. M. Science 1971, 171.1008-13. (6) Smith, S. V. Limnol. Oceanogr. 1984, 29,114WO. (7) Hecky, R. E.; Kilham, P. Limnol. Oceanogr. 1988.33, 796822. (8) Howes, 9. L.; Taylor. C. D. “Nutrient Regime of New Bedford Outer Harbor”; final technical report to Camp, Dresser, and McKee, Boston, MA; City of New Bedford; and US. EPA Woods Hole Oceanographic Institution: Woods Hole, MA, 1989. (9) Teal, 1. M.; Peterson, S. B. I. Water Pollut. Control Fed. 1991, 63, 84-89. (10) Septage TreotmentandDisposal; US. Environmental Protection Agency. Municipal Environmental Research Laboratory.US.Government Printing Office: Washington. DC, 1984; EPA 625/6-84-009. (11) Giggey, M. D.; Brantner. K. A. Presented at Sludge Management Conference of the Water Environment Federation, Topsham, ME, July 1992
John M. Teal is o senior scientist at Woods Hole Oceanographic Institution in Woods Hole, MA, and serves os vicec h a i r m a n of t h e board of the Conservation Law Foundation of N e w England. He begm his research on wetlands systems at the Universityof Georgia in the 1950s. He and his colleagues have conducted long-term experiments with salt marshes, including manipulation of nutrient and pollutant loadings, the impact of oil spills, and the degradation of hydmcnrbons in coastal wetlands. He has also researched the influence of pollutants on groundwater.
Susan B. Peterson is president of Ecological Engineering Associates (EEAI of Marion,MA. An anthropologist b y training, she spent the 1970s and 1980s at Woods Hole, focusing on coastal policy, marine economics,-ond envimnmental management. She was subseReferences quently on the staff of Boston University and a e o n Arks Intemotlonal. She rind (I] canter. L. c.; Knox. K. w. Septic Tank her partnes eAtablished EEA in 1988 to system ~ f / on~Cmundl,.oter~uol. ~ t ~ rommercialize natural systems technolily: Lewis Publishers. Chelsea. MI. ogy for trwting polluted water. 1985. pp. 45-101. Environ. Sci. Technol., Vol. 27,No. 1, 1993 37
