Performance of Peat Filters

Performance of Peat Filters

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ABSTRACT PERFORMANCE OF PEAT FILTERS IN THE TREATMENT OF DOMESTIC WASTEWATER IN MINNESOTA S. D. Monson Geerts, B. McCarthy, R. Axler, J. Henneck, S. Heger Christopherson, J. Crosby, and M. Guite*

ABSTRACT Approximately 500,000 Minnesota residences rely on the use of onsite wastewater treatment systems, and 50-70% of these are not in compliance with state codes or are failing hydraulically due to restrictive site and soil conditions. Multi-interest research sites were established in northern and southern Minnesota in the fall of 1995 to design, construct, and monitor the performance of advanced pretreatment systems. The pretreatment systems were designed to treat septic tank effluent from a single family home, ~950 liters/day (250 gal/day) to meet secondary treatment levels of 25 mg/L total suspended solids (TSS), 30 mg/L biochemical oxygen demand (BOD5), and 200 cfu/100mL fecal coliform bacteria. Replicate in-ground (lined excavation) intermittent peat filters with gravity distribution, experienced hydraulic failure after 15 months at the northern site, but were later modified with pressure distribution, and have operated successfully since. Removal efficiencies are 98% TSS, >99% BOD5, >99.99% fecal coliform bacteria, >42% TP, and >17% TN. Similarly constructed in-ground intermittent peat filters at the southern site also experienced hydraulic failure. However, a peat filter with pressure distribution is still in operation and functioning (since 1996) in a partial anaerobic condition. During the summer and winter of 1998, the in-ground intermittent peat filters at the northern site were spiked with Salmonella choleraesuis, and had an overall 9 log removal efficiency. Modular recirculating peat filters (Puraflo®) were installed at the northern site in the summer of 1998 to compare a proprietary Irish peat with a Bord Na Móna specified proprietary Minnesota peat. Removal efficiencies for both have been >92% TSS, >96% BOD5, >99% fecal coliform bacteria, 3-20% TP, and 29-41% TN. Both the in-ground and the modular peat filters are performing well and consistently exceeding secondary levels of treatment. KEYWORDS. domestic wastewater, peat, filtration, secondary-treatment.

INTRODUCTION The treatment and dispersal of domestic wastewater in unsewered areas is a significant public health and environmental issue. More than 500,000 residences and commercial establishments in Minnesota rely on the use of onsite wastewater treatment systems, and according to the Minnesota Pollution Control Agency (MPCA, 1994), 50-70% are not in compliance with state standards. A University/multi-industry/local, state, and federal agency project was established in 1995. Two research sites were created, one near Duluth, Minnesota, at the Northeast Regional Correction Center (NERCC) and the other near Lake Washington in Le Sueur County. The purpose of the research sites was to design, construct, operate, and monitor the performance of alternative onsite treatment systems for domestic (residential strength) wastewater. Individual treatment systems were selected that were considered not only technically feasible and used widely but also likely to succeed through Minnesota winters. These include both in-ground (lined excavation) intermittent peat and sand filters, constructed wetlands, drip technology (NERCC), modular recirculating peat filters (NERCC), textile filter (NERCC), and standard drainfield trenches. The effectiveness of the onsite systems is determined by their ability to remove pollutants year-round from wastewater. This paper presents the operation and performance data of the peat filter systems at both research sites.

MATERIALS AND METHODS The research facilities are unique in that the treatment systems use the same wastewater at similar daily flows and are replicated, allowing for side-by-side comparison of performance of both alternative and standard onsite technology. All the systems were designed to achieve a secondary level of treatment of 25 mg/L total suspended solids (TSS), 30 mg/L biochemical oxygen demand (BOD5), and 200 cfu/100 mL fecal coliform bacteria.

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In-Ground Peat Filters The in-ground intermittent peat filters at NERCC were completed in October 1995, and designed to treat 0.95 m3 (250 gal/day) of septic tank effluent (STE) at a hydraulic loading rate of 3.3 cm/d (1.3 in/d). The peat was air-dried, unscreened Sphagnum peat from Minnesota Sphagnum Inc. (MSI), similar to material used in the State of Maine (Brooks et al., 1984). The filters are lined excavations 7.0 m L x 4.1 m W x 1.4 m D (23' L x 13.5' W x 4.5' D). Peat was placed in ~30 cm (12") lifts and compacted using snowshoes. The gravity distribution network consisted of four 10 cm (4") diameter perforated laterals imbedded in pea gravel trenches. Each filter was dosed with 237 L (~60 gallon) STE every 4 hours. Two similarly designed peat filters, hydraulically loaded at the same rate, were installed at the Lake Washington site in September 1996. Both of the Lake Washington peat filters were constructed using gravel to level the distribution networks, however, one was set up with gravity and the other with a pressure distribution system. Modular Peat Filters The modular peat filter units (Puraflo® by Bord Na Móna), were installed in July 1998 and can be operated in intermittent or in recirculation mode. Two Puraflo® systems were installed, each comprising 3 modules, 2.2 m L x 1.4 m W x 0.76 m D (85" L x 55" W x 18" D) in series, one filled with a proprietary Irish peat and the other with a Bord Na Móna specified proprietary MSI peat (Fig. 1). The peat was compacted by foot in lifts of 30-45 cm (12-18") and covered. Each system was constructed with a dose/recirculation tank with a 1:1 recirculating ratio, dosed every 1.5 hours at a hydraulic loading rate of 21 cm/d (8 in/d) and a total daily discharge of 0.95 m3 (250 gal). All systems are monitored every three weeks for TSS, BOD5, fecal coliform bacteria and total phosphorus (TP). Samples are analyzed (APHA, 1995) at the Western Lake Superior Sanitary District (WLSSD). All nitrogen tests [total nitrogen (TN), dissolved-N, ammonia-N, and nitrate-N], pH, alkalinity, chloride and major ions are analyzed at the Natural Resources Research Institute (NRRI) (APHA, 1995; Owen and Axler, 1991).

RESULTS AND DISCUSSION This section highlights winter and summer performance of the peat filters. Influent wastewater strength at NERCC was slightly higher than typical residential strength STE, while influent strengths at Lake Washington were slightly lower. Crites and Tchobanoglaus, (1998) give typical septic tank effluent parameter concentrations for systems without an effluent filter, BOD5 (150-250 mg/L), TSS (40-140 mg/L), TP (12-20 mg/L) and TN (50-90 mg/L).

Figure 1. Schematic of the Modular Peat Filter Systems at the NERCC Research Facility.

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In-Ground Peat Filters Gravity Distribution Performance of the NERCC peat filters was generally good for removal of TSS, BOD5 and fecal coliforms (Table 1). Effluent TSS averaged 3.2 mg/L (~92% removal), while BOD5 ranged from 5-23 mg/L (>90%removal). Fecal coliform levels were consistently less than the target standard of 200 cfu/100 mL, and often below detection i.e. 99.9%. These results are similar to other studies conducted in both warm and cold climates (White et al., 1995; Couillard, 1994). TP removal was ~34% with effluent values of 6-7 mg/L. The published literature indicates that peat filters are quite variable in removing phosphorus, and can range between 10% and 80% (Viraraghaven and Rana, 1991; Brooks et al., 1984). Contrary to what would normally be expected, TN removal was actually higher during winter (62%) than summer (30%), and was probably due to a complex network of coexisting aerobic and anaerobic conditions within the filters during the 1996/97 winter. Ammonium removal averaged ~60%, some of which was nitrified, as evidenced by 4-20 mg/L nitrate-N in the effluent. Table 1. Performance of In-Ground Peat Filters with Gravity Distribution. NERCC In-Ground Peat Filters - Gravity Distribution Parameter WINTER INFLOWa Effluent Q (L/d) TSS (mg/L) BOD5 (mg/L) TP (mg/L) TN (mg/L) NH4-N (mg/L) NO3-N (mg/L) fecal coliformsd

38 206 10 75 64 0.02 1.2x105

b

(787 L/d) 2.9 23 7.2 27 20 4.0 13

% - Removal

92 90 32 62 67 (part. nitrif.) 99.90

SUMMER c

Effluentb (522 L/d) 3.4 4.8 6.2 49 28 20 8.0

%Removalc 91 97 37 30 53 (part. nitrif.) 99.99

Total number of sampling events = 10 winter, 10 summer, February 1996 through April 1997; a Average during the time period; b Average seasonal values; c Percent removal based on Effluent: [(inflow-outflow)/inflow] x 100 = % removed; d The geometric mean colony forming units (cfu) per 100ml. Overall, the data indicate that the peat filters were operating in a partially anaerobic condition, indicated by less than complete nitrification. After ~15 months, the NERCC peat filters with gravity distribution, failed hydraulically. The peat filter with gravity distribution at Lake Washington hydraulically failed after 6 months, thus the performance data were not included in this paper. Suspected reasons include: use of a gravity distribution network imbedded in pea rock caused ponding in the rock due to a biomat/slime layer at the rock/peat interface; excess compaction of the peat during construction and from the weight of the pea rock and wastewater that ponded in the network; and excessive fines in the Sphagnum peat (McCarthy et al., 1997).

Pressure Distribution During re-construction the peat was replaced with coarse peat from Minnesota with minimal compaction. A pressure distribution system was installed and programmed to dose 158 liters (~40 gal) of STE every 6 hours (Fig 2). Performance has been excellent since re-construction (McCarthy et al., 1998b; Monson Geerts and McCarthy, 1999) (Table 2). Effluent TSS averaged 1 mg/L (98% removal), and BOD5 averaged 99 48 17 84 (nitrification) >99.99

(712 L/d) 1.1 1.5 9.4 72 12 60 8.8

%Removalc 98 >99 42 20 86 (nitrification) >99.99

Total number of sampling events = 23 winter, 20 summer, November 1997 through June 2000; a Average during the time period; b Average seasonal values; c Percent removal based on Effluent: [(inflow-outflow)/inflow] x 100 = % removed; d The geometric mean colony forming units (cfu) per 100ml.

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Table 3. Performance of Lake Washington In-Ground Peat Filter with Pressure Distribution. Lake Washington In-Ground Peat Filters - Pressure Distribution WINTER SUMMER Parameter INFLOWa Effluentb

%Removal

Q (L/d) TSS (mg/L) 46 BOD5 (mg/L) 127 TP (mg/L) 6.2 TN (mg/L) 38 31 NH4-N (mg/L) NO3-N 0.57 (mg/L) fecal 1.1x106 d coliforms

c

(1316 L/d) 12 17 2.9 7.3 3.8

68 82 51 75 87

5.4

(part. nitrif.) 95.6

6.3x103

Effluentb (1018 L/d) 8.2 11 3.2 20 6.2 5.5 3.4x104

%Removalc

82 88 48 43 78 (part. nitrif.) 96.8

Total number of sampling events = 30 winter, 17 summer, November 1996 through June 2000; a Average during the time period; b Average seasonal values; c Percent removal based on Effluent: [(inflow-outflow)/inflow] x 100 = % removed; d The geometric mean colony forming units (cfu) per 100ml. Modular Peat Filters The modular recirculating Puraflo® systems operated through the first winter, although thermocouple readings indicated temperatures below freezing at the distribution network in January 1999. Minimal snowfall had occurred throughout the winter, a very atypical situation for northern Minnesota. The modules were then insulated with polystyrene and remained above freezing the rest of winter. In April 1999 performance decreased, and ponding was observed in each system. We speculate that cold temperatures, coupled with high hydraulic and organic (BOD5 > 300 mg/L) loading rates, promoted the formation of a biomat/slime layer just below the distribution network in each module. The systems were rested for 10 days while the upper 30-45 cm (12-18") of peat was turned, aerated and allowed to dry. The standard Irish peat fully recovered. However, the Minnesota peat ponded a second time in May, and it was re-constructed in August 1999. Modifications to the filter include: additional rock in the underdrain system to prevent the migration of fines; less compaction of the peat during placement; and inclusion of insulated covers on both systems. The mode of operation was changed from recirculating to intermittent, and both systems have performed well in the intermittent mode 0.95 m3/day (250 gal/day) since mid-April 1999.

Standard Irish Peat Medium The Irish medium achieved secondary standards for both TSS and BOD5 (Tables 4 & 5). Average TSS was typically £2.4 mg/L (>96% removal) with a slightly higher TSS (6.5 mg/L) during the

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Table 4. Performance of Modular Peat Filters - Irish Medium (recirculating mode). NERCC Modular (Puraflo®) Recirculating Peat Filters - Irish Medium WINTER SUMMER Parameter INFLOWa

Q (L/d) TSS (mg/L) BOD5 (mg/L) TP (mg/L) TN (mg/L) NH4-N (mg/L) NO3-N (mg/L) fecal coliformsd

52 301 16 99 90 0.03 6.4x105

Effluentb

% - Removalc

(931 L/d) 6.5 13 15 69 33 34 3302

87 96 7 33 65 (nitrification) 99.65

Effluentb

%Removalc

(689 L/d) 1.4 97 11 96 12 20 57 37 4.4 95 55 (nitrification) 84 99.97

Total number of sampling events = 8 winter, 6 summer, July 1998 through June 1999; a Average during the time period; b Average seasonal values; c Percent removal based on Effluent: [(inflow-outflow)/inflow] x 100 = % removed; d The geometric mean colony forming units (cfu) per 100ml. Table 5. Performance of Modular Peat Filters - Irish Medium (intermittent mode). NERCC Modular (Puraflo®) Intermittent Peat Filters - Irish Medium WINTER Parameter INFLOWa Effluent Q (L/d) TSS (mg/L) BOD5 (mg/L) TP (mg/L) TN (mg/L) NH4-N (mg/L) NO3-N (mg/L) fecal coliformsd

58 296 16 96 82 0.02 6.6x105

b

(1083 L/d) 1.3 3.7 13 56 6.1 50 501

% - Removal

c

98 99 11 41 92 (nitrification) 99.95

SUMMER

Effluentb

%Removalc

(1215 L/d) 2.4 96 3.8 99 19 3 70 30 2.1 98 68 (nitrification) 43 99.99

Total number of sampling events = 9 winter, 6 summer, June 1999 through June 2000; a Average during the time period; b Average seasonal values; c Percent removal based on Effluent: [(inflow-outflow)/inflow] x 100 = % removed; d The geometric mean colony forming units (cfu) per 100ml. 1998-99 winter while operating as a recirculating filter. Average BOD5 ranged from 12 mg/L (96% removal) in recirculating mode, to 3.8 mg/L (99% removal) as an intermittent filter. Secondary standards were achieved for fecal coliform bacteria during the summer (> 99.9% removal) but exceeded the standard during winter operation in both modes (501-3,302 cfu/100 mL). Average TP removal began at a high of 20% and, as expected, has declined steadily over the two years of operation. Ammonia removal rates were fairly consistent (92-98%) except during winter recirculation mode (65%), which may be attributed to the freezing/ponding problems that were encountered. This is also evident in the nitrate data. However, for reasons unknown the TN data is not consistent, but alternatively, shows a greater percent TN removed during the winter in intermittent mode.

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Minnesota Peat Medium The Minnesota peat also achieved secondary standards for TSS and BOD5 (Tables 6 & 7). Average Table 6. Performance of Modular Peat Filters - Minnesota Medium (recirculating mode). NERCC Modular (Puraflo®) Recirculating Peat Filters - Minnesota Medium WINTER SUMMER Parameter INFLOWa


52 301 16 99 90 0.03 6.4x105

Effluentb

% - Removalc

(1120 L/d) 7.4 20 14 72 55 19 29,603

86 94 15 30 41 (part. nitrif.) 96.69

Effluentb

%Removalc

(662 L/d) 3.3 94 9.0 97 12 19 51 43 19 78 33 (nitrification) 238 99.94

Total number of sampling events = 8 winter, 6 summer, July 1998 through June 1999; a Average during the time period; b Average seasonal values; c Percent removal based on Effluent: [(inflow-outflow)/inflow] x 100 = % removed; d The geometric mean colony forming units (cfu) per 100ml. Table 7. Performance of Modular Peat Filters - Minnesota Medium (intermittent mode). NERCC Modular (Puraflo®) Intermittent Peat Filters - Minnesota Medium WINTER SUMMER Parameter INFLOWa


58 296 16 96 82 0.02 6.6x105

Effluentb

% - Removalc

(1086 L/d) 4.3 14 11 58 11 47 5081

92 96 20 36 87 (nitrification) 99.49

Effluentb

%Removalc

(1079 L/d) 2.5 96 4.5 98 14 14 66 29 25 68 43 (nitrification) 2023 99.50

Total number of sampling events = 8 winter, 2 summer, September 1999 through June 2000; a Average during the time period; b Average seasonal values; c Percent removal based on Effluent: [(inflow-outflow)/inflow] x 100 = % removed; d The geometric mean colony forming units (cfu) per 100ml. TSS ranged between 2.5-7.4 mg/L (92% removal). BOD5 removal was better during the summer, and ranged between 4.5-9.0 mg/L (~98% removal) as compared to winter performance, which ranged between 14-20 mg/L (~95% removal). On average, secondary treatment standards were not met for fecal coliforms with 238-29,603 cfu/100mL (96-99% removal) in the effluent. Unlike the Irish medium, average TP removal remained somewhat constant over the two years of operation. Similar to the Irish medium, TN removal was more efficient for the recirculation mode during the summer and for the intermittent mode during the winter. The average ammonium concentration of 55 mg/L and nitrate concentration of 19 mg/L during the winter recirculation mode confirm that the system was partially anaerobic. However, performance data from the intermittent mode indicate that the system was performing aerobically.

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SUMMARY AND CONCLUSIONS Some minor problems were encountered with peat filter designs, but appear to be remedied. The type of peat used in a peat filter is critical. Highly porous, coarse, fibrous peat is preferable. Gravel used to level the distribution piping within the filter added much weight to the peat below it. This weight appears to compact the peat and decrease the porosity resulting in biomat formation at the peat/gravel interface, which eventually resulted in hydraulic failure. Pressure distribution of the septic tank effluent using time-dosed intervals increased the performance and longevity of the filters. The Minnesota peat used in the in-ground intermittent peat filters and one of the Puraflo® modular systems, performed best when placed in the filter with minimal compaction, provided the distribution network is supported and remains level. Adequate insulation also proved to be a critical element in the winter performance of peat filters. In the northern Minnesota climate, it is recommended that there be a minimum of 30 cm (12") of peat above the distribution network and/or, in the case of modular peat filters an insulated cover for maintaining suitable temperatures. Despite the problems, the research site at NERCC demonstrated that peat filters can perform well and achieve secondary effluent standards year-round in Northern Minnesota.

ACKNOWLEDGMENTS This project depended on the support of many dedicated people from Natural Resources Research Institute, Western Lake Superior Sanitary District, Northeast Regional Correction Center, St. Louis County, Bord Na Móna, Minnesota Sphagnum, Inc., contractors and other businesses from the private sector, and numerous governmental agencies. Grant support was provided by the Legislative Commission on Minnesota Resources (LCMR) through the Minnesota Pollution Control Agency (MPCA), Minnesota Technology Inc. (MTI), Saint Louis County, and the WLSSD.

REFERENCES 1. APHA, 1995. Standards methods for the examination of water and wastewater. American Public Health Association, Washington, D.C. 2. Brooks, J.L., C.A. Rock, and R.A. Struchtemeyer. 1984. Use of peat for on-site wastewater treatment: field studies. J. Environ. Qual. 13:524-530. 3. Crites, R. and G. Tchobanoglous. 1998. Small and Decentralized Wastewater Management Systems. McGraw-Hill Companies, Inc., 1084 p. 4. Couillard, D. 1994. The use of peat in wastewater treatment. Wat. Res. 28:1261-1274. 5. McCarthy, B., R. Axler, S. Monson Geerts, J. Henneck, J. Crosby, D. Nordman, P. Weidman and T. Hagen. 1997. Development of alternative on-site treatment systems for wastewater treatment: A demonstration project for Northern Minnesota. NRRI Technical Report NRRI/TR -97/10, Natural Resources Research Institute, University of Minnesota, Duluth, MN. 6. McCarthy, B., R. Axler, S. Monson Geerts, J. Henneck, D. Nordman, J. Crosby, P. Weidman, 1998b. Performance of alternative treatment systems in Northern Minnesota. Pages 446 -457, In: Onsite Wastewater Treatment, Proceedings of the Eighth International Symposium on Individual and Small Community Sewage Systems, Orlando, FL, March 1998, American Society of Agricultural Engineers, St. Joseph, MI. 7. Monson Geerts, S., B. McCarthy, 1999. Wastewater Treatment by Peat Filters. Focus 10,000, 10:16-20. 8. MPCA. 1994 (revised 1996). Individual Sewage Treatment System Standards: Chapter 7080 MN Rules. Minnesota Pollution Control Agency, Water Quality Div., St. Paul, MN 55155. 9. Owen, C.J. and R. Axler. 1991. Analytical Chemistry and Quality Assurance Manual. NRRI/TR-91/05 (1996 revision). Natural Resources Research Institute, Duluth, Minnesota. 10. Pundsack, J.P., R. Axler, R. Hicks, J. Henneck, D. Nordman and B. McCarthy. 2000. Seasonal Pathogen Removal by Alternative On-Site Wastewater Treatment Systems. Water Environ. Research (In Press). 11. Pundsack, J.P. 2000. Pathogen removal by alternative on-site systems for treatment of single home wastewater flows. M.S. thesis (Water Resource Science), University of Minnesota. July 2000. 12. Viraraghaven, T. and S.M. Rana. 1991. Treatment of septic tank effluent in a peat filter. Inter. J. Environmental Studies. 37:213-225. 13. White, K.D., L.A. Byrd, S.C. Robertson, J.P. O’Driscoll and T. King. 1995. Evaluation of peat filters for onsite sewage management. Environ. Health Non. 1995: 11-15.

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