Natural Wastewater Treatment Systems - Chapter 10 doc

493 10 On-Site Wastewater Systems Effluent disposal options for on-site systems range from soil absorption in con- ventional gravity leachfields to water reuse after high-tech membrane treatment. Individual on-site systems are the most prevalent wastewater management sys- tems in the country. This chapter describes the various types of on-site wastewater systems, wastewater disposal options, site evaluation and assessment procedures, cumulative areal nitrogen loadings, nutrient removal alternatives, disposal of variously treated effluents in soils, design criteria for on-site disposal alternatives, design criteria for on-site reuse alternatives, correction of failed systems, and role of on-site management systems. 10.1 TYPES OF ON-SITE SYSTEMS While many types of on-site systems exist, most involve some variation of subsurface disposal of septic tank effluent. The four major categories of on-site systems are: • Conventional on-site systems •Modified conventional on-site systems • Alternative on-site systems • On-site systems with additional treatment The most common on-site system is the conventional on-site system that consists of a septic tank and a soil absorption system (see Figure 10.1). The septic tank is the wastewater pretreatment unit used prior to on-site treatment and disposal. Modified conventional on-site systems include shallow trenches and pressure- dosed systems. Alternative on-site disposal systems include mounds, evapotrans- piration systems, and constructed wetlands. Additional treatment of septic tank effluent is sometimes needed, and intermittent and recirculating granular-medium filters are often the economical choice. Where further nitrogen removal is required, one or more of the alternatives for nitrogen removal (see Section 10.4) may be considered. The types of disposal and reuse systems used for individual on-site systems are presented in Table 10.1. DK804X_C010.fm Page 493 Friday, July 1, 2005 4:52 PM © 2006 by Taylor & Francis Group, LLC 494 Natural Wastewater Treatment Systems 10.2 EFFLUENT DISPOSAL AND REUSE OPTIONS Alternative infiltration systems (presented in Table 10.2) have been developed to overcome restrictive conditions such as: •Very rapidly permeable soils •Very slowly permeable soils • Shallow soil over bedrock • Shallow groundwater • Steep slopes • Groundwater quality restrictions • Limited space The alternatives for reuse of on-site system effluent include drip irrigation, spray irrigation, groundwater recharge, and toilet flushing. Drip irrigation is becoming more popular for water reuse and is described in this chapter. Spray irrigation is more suited to larger flows (commercial, industrial, and small community flows) and is described in detail in Chapter 8. Groundwater recharge, which is used in areas of deep permeable soils, is also described in Chapter 8. 10.3 SITE EVALUATION AND ASSESSMENT The process of selecting a suitable on-site location for on-site disposal involves multiple steps of identification, reconnaissance, and assessment. The process begins with a thorough examination of the soil characteristics, which include permeability, depth, texture, structure, and pore sizes. The nature of the soil profile and the soil permeability are of critical concern in the evaluation and assessment of the site. Other important aspects of the site are the depth to groundwater, site FIGURE 10.1 Typical cross-section through conventional soil absorption system. Native soil backfill Fabric or building paper 6 in. minimum 12 in. minimum 4-in. distribution pipe Side wall absorption area (both sides) 18–24 in. min 36-in. max 2-in. minimum rock over pipe 6-in. minimum rock under pipe .75- to 2.5-in diameter washed drainrock DK804X_C010.fm Page 494 Friday, July 1, 2005 4:52 PM © 2006 by Taylor & Francis Group, LLC On-Site Wastewater Systems 495 slope, existing landscape and vegetation, and surface drainage features. After a potential site has been located, the site evaluation and assessment proceeds, generally in two phases: preliminary site evaluation and detailed site assessment. TABLE 10.1 Types of On-Site Wastewater Disposal/Reuse Systems Disposal/Reuse System Remarks Conventional Systems Gravity leachfields/conventional trench Gravity absorption beds Most common system — Modified Conventional Systems Gravity leachfields: Deep trench To get below restrictive layers Shallow trench Enhanced soil treatment Pressure-dosed: Conventional trench To reach uphill fields Shallow trench Uphill and shallow sites Drip application Following additional treatment of septic tank effluent; to optimize use of available land area Alternative Systems Sand-filled trenches Added treatment At-grade systems Less expensive than mounds Fill systems Import soil Mound Systems Evapotranspiration systems Zero discharge Evaporation ponds See Chapter 4 Constructed wetlands Requires a discharge or subsequent infiltration (see Chapter 7) Reuse Systems Drip irrigation Usually follows added treatment Spray irrigation Requires disinfection Graywater reuse — Other Systems Holding tanks Seasonal use alternative Surface water discharge Allowed in some states following added treatment DK804X_C010.fm Page 495 Friday, July 1, 2005 4:52 PM © 2006 by Taylor & Francis Group, LLC TABLE 10.2 Appropriate On-Site Disposal Methods To Overcome Site Constraints Method Soil Permeability Bedrock Groundwater Slope Small Lot SizeVery Rapid Moderately Rapid Very Slow Shallow Deep Shallow Deep 0–5% >5% Trenches • • • •••• Beds •••••• Pits • • •••• Mounds • • • ••••••• Fill systems • • • ••••••• Sand-lined trenches and beds ••• • •••• Drained systems • • • • • Evaporation ponds • • • ••••• ET beds • • • ••••• ETA beds • • • •••• Spray irrigation • • • •••••• Drip irrigation • • • •••••• Note: The symbol • indicates appropriate system; ET, evapotranspiration; ETA, evapotranspiration–absorption. DK804X_C010.fm Page 496 Friday, July 1, 2005 4:52 PM © 2006 by Taylor & Francis Group, LLC On-Site Wastewater Systems 497 10.3.1 P RELIMINARY S ITE E VALUATION The initial step in conducting a preliminary site evaluation is to determine the current and proposed land use, the expected flow and characteristics of the wastewater, and to observe the site characteristics. The next step is to gather information on the following characteristics: • Soil depth • Soil permeability (general or qualitative) • Site slope • Site drainage • Existence of streams, drainage courses, or wetlands • Existing and proposed structures •Water wells • Zoning •Vegetation and landscape 10.3.2 A PPLICABLE R EGULATIONS When the pertinent data have been collected, the local regulatory agency should be contacted to determine the regulatory requirements. The tests required for the phase 2 investigation, which can include identifying depth to groundwater during the wettest period of the year and permeability tests to determine water absorption rates, can also be determined at this time. A list of typical regulatory factors for on-site disposal is presented in Table 10.3. TABLE 10.3 Typical Regulatory Factors in On-Site Systems Factor Unit Typical Value Setback distances (horizontal, separation from wells, springs, surface waters, escarpments, site boundaries, buildings) ft (See Table 10.12) Maximum slope for on-site disposal field % 25-30 Soil characteristics: Depth ft 2 Percolation rate min/in. >1 to <120 Minimum depth to groundwater ft 3 Septic tank (minimum size) gal 750 Maximum hydraulic loading rates for leachfields gal/ft 2 ·d 1.5 Maximum loading rates for sand filters gal/ft 2 ·d 1.2 DK804X_C010.fm Page 497 Friday, July 1, 2005 4:52 PM © 2006 by Taylor & Francis Group, LLC 498 Natural Wastewater Treatment Systems 10.3.3 D ETAILED S ITE A SSESSMENT The important parameters that require field investigation are soil type, structure, permeability, and depth, as well as depth to groundwater. The use of backhoe pits, soil augers, piezometers, and percolation tests may be required to characterize the soil. Backhoe pits are useful to allow a detailed examination of the soil profile for soil texture, color, degree of saturation, horizons, discontinuities, and restrictions to water movement. Soil augers are useful in determining the soil depth, soil type, and soil moisture, and many hand borings can be made across a site prior to the siting of a backhoe pit location. Piezometers are occasionally required by regula- tory agencies to determine the level and fluctuation of groundwater. In most parts of the country, the results of percolation tests are used to determine the required size of the soil absorption area. The allowable hydraulic loading rate for the soil absorption system is determined from a curve or table that relates allowable loading rates to the measured percolation rate. A typical curve relating percolation rate to hydraulic loading rate for subsurface soil absorp- tion systems is shown in Figure 10.2. In the percolation test, test holes that vary in diameter from 4 to 12 in. (100 to 300 mm) are bored in the location of the proposed soil absorption area. The bottom of the test hole is placed at the same depth as the proposed bottom of the absorption area. Prior to measuring the percolation rate, the hole should be soaked for a period of 24 hr. Tests and acceptable procedures used by local regulatory agencies should be checked prior to site investigations. FIGURE 10.2 Percolation rate vs. hydraulic loading rate for soil absorption systems. (From Winneberger, J.H.T., Septic-Tank Systems: A Consultant’s Toolkit . Vol. 1. Subsurface Disposal of Septic-Tank Effluents , Butterworth, Boston, MA, 1984. With permission.) Ryonʼs line used Ryonʼs line including all points USPHS Study, troublefree system USPHS Study, troubled system Time for water surface to fall 1 inch (minutes) Hydraulic loading rate (gal/ft 2 -d) 0 10 20 30 40 50 60 70 80 90 100 6 5 4 3 2 1 0 DK804X_C010.fm Page 498 Friday, July 1, 2005 4:52 PM © 2006 by Taylor & Francis Group, LLC On-Site Wastewater Systems 499 Although used commonly, the percolation test results, because of the nature of the test, are not related to the performance of the actual leachfields. Many agencies and states are abandoning the test in favor of detailed soil profile evaluations. The percolation test is only useful in identifying soil permeabilities that are very rapid or very slow. Percolation tests should not be used as the sole basis for design of soil absorption systems because of the inherent inaccuracies. 10.3.4 H YDRAULIC A SSIMILATIVE C APACITY For facilities that are designed for larger flows than those generated by individual households or for sites where the hydraulic capacity is borderline within the local regulations, a shallow trench pump-in test or a basin infiltration test can be used. The absorption test has been developed for wastewater disposal (Wert, 1997). This procedure allows an experienced person to determine the site absorption capacity. In the shallow trench pump-in test, a trench 6 to 10 ft (2 to 3 m) long is excavated to the depth of the proposed disposal trenches. Gravel is placed in a wooden box in the trench to simulate a leachfield condition. A constant head is maintained using a pump, water meter, and float. The soil acceptance rate is then calculated by measuring the amount of water that is pumped into the soil over a period of 2 to 8 d. 10.4 CUMULATIVE AREAL NITROGEN LOADINGS As described in Chapter 3, nitrogen forms can be transformed when released to the environment. Because the oxidized form of nitrogen, nitrate nitrogen, is a public health concern in drinking water supplies, the areal loading of nitrogen is important. 10.4.1 N ITROGEN L OADING FROM C ONVENTIONAL E FFLUENT L EACHFIELDS The nitrogen loading from conventional leachfields depends on the density of housing and the nitrogen in the applied effluent. The impact of the nitrate nitrogen on groundwater quality depends on the nitrogen loading, the water balance, and the background concentration of nitrate nitrogen. To determine the nitrogen loading, the following procedure is suggested: 1. Determine the wastewater loading rate. The unit generation factor is multiplied by the density of the units per acre; for example, 150- gal/household × 4 houses per acre yields 600 gal/d·ac. 2. Determine the nitrogen concentration in the applied effluent (use 60 mg/L). 3. Calculate the nitrogen loading. Multiply the nitrogen concentration by the wastewater loading: Nitrogen loading (lb/ac·d) = L × N c × C × 10 –6 (10.1) DK804X_C010.fm Page 499 Friday, July 1, 2005 4:52 PM © 2006 by Taylor & Francis Group, LLC 500 Natural Wastewater Treatment Systems where L =Wastewater loading (gal/ac·d). N c = Nitrogen concentration (mg/L). C=8.34 lb/gal. 10 –6 =Parts per million = mg/L. 4. In this example, Nitrogen loading = (600 gal/ac·d)(60 mg/L)(8.34)(10 –6 ) = 0.30 lb/ac·d (135 gal/ac·d) 10.4.2 C UMULATIVE N ITROGEN L OADINGS The loadings of nitrate nitrogen to the groundwater are reduced by denitrification in the soil column. As indicated in Chapter 8, denitrification depends on the carbon available in the soil or the percolating wastewater and on the soil perco- lation rate. For sandy, well-drained soils, the denitrification fraction is 15%. For heavier soils or where high groundwater or slowly permeable subsoils reduce the rate of percolation, the denitrification fraction can be estimated at 25%. The percolate nitrate concentration can be calculated from Equation 10.2: N p = N c (1 – f ) (10.2) where N p = Nitrate nitrogen in the leachfield percolate (mg/L). N c = Nitrogen concentration in the applied effluent (mg/L). f = Denitrification decimal fraction (0.15 to 0.25). Example 10.1. Nitrogen Loading Rate in On-Site Systems A local environmental health ordinance limits the application of septic tank effluent on an areal basis to 45 g/ac·d. Determine the housing density with conventional septic tank effluent–soil absorption systems that will comply with the ordinance. Assume a total nitrogen content in the septic tank effluent of 60 mg/L and a household wastewater generation of 175 gal/d. Solution 1. Determine the acceptable loading rate in lb/ac·d: N L = 45 g/ac·d × 1/454 g/lb = 0.099 lb/ac·d 2. Calculate the corresponding wastewater application rate using Equation 10.1: L = Nitrogen loading/(nitrogen concentration × 8.34)(10 –6 ) L = 0.099 lb/ac·d/(60 mg/L × 8.34 lb/gal)(10 –6 ) L = 197.8 gal/ac·d DK804X_C010.fm Page 500 Friday, July 1, 2005 4:52 PM © 2006 by Taylor & Francis Group, LLC On-Site Wastewater Systems 501 3. Determine the number of households per acre: Households per acre = L /175 gal/d = 1.13 4. Calculate the minimum lot size for compliance: Lot size = 1/1.13 = 0.88 ac Comment This would be a very conservative ordinance. If a 25% denitrification fraction were recognized in the ordinance, the nitrogen loading rate would be increased to 60 g/ac·d. 10.5 ALTERNATIVE NUTRIENT REMOVAL PROCESSES Alternative nutrient removal processes have been and continue to be developed for the cost-effective control of nutrients from on-site systems. Nitrogen removal is the most critical of the nutrients because nitrogen can have public health effects as well as eutrophication and toxicological impacts. A large group of attached growth and suspended growth biological systems are available for pretreatment (Tchobanoglous et al., 2003). A listing of attached growth bioreactors used with on-site systems is presented in Table 10.4. 10.5.1 N ITROGEN R EMOVAL Removal of nitrogen is a critical issue in most on-site disposal systems. On-site nitrogen removal processes include intermittent sand filters and recirculating granular medium filters, as well as septic tanks with attached growth reactors (internal trickling filters in septic tanks). 10.5.1.1 Intermittent Sand Filters As described in Chapter 5, intermittent sand filters are shallow beds (2 ft thick) of fine to medium sand with a surface distribution system and an underdrain system. In the late 1880s, many Massachusetts communities used the intermittent sand filter (ISF) to treat septic tanks effluent (Mancl and Peeples, 1991). The ISFs were the forerunners of rapid infiltration and vertical flow wetlands, with hydraulic loading rates of 0.48 to 2.77 gal/d·ft 2 (19 to 113 mm/d). A typical ISF is shown in Figure 10.3. Septic tank effluent is applied inter- mittently to the surface of the sand bed. The treated water is collected an under- drain system that is located at the bottom of the filter. Intermittent filters are either open or buried, but the majority of on-site ISFs have buried distribution systems. The treatment performance of ISF systems is presented in Table 10.5. Suspended solids and bacteria are removed by filtration and sedimentation. BOD and ammo- nia are removed by bacterial oxidation. Intermittent application and venting of DK804X_C010.fm Page 501 Friday, July 1, 2005 4:52 PM © 2006 by Taylor & Francis Group, LLC 502 Natural Wastewater Treatment Systems the underdrains help to maintain aerobic conditions within the filter. Denitrifica- tion can be enhanced by flooding the underdrains. The key design factors for ISFs are sand size, sand depth, hydraulic loading rate, and dosing frequency. The smaller sand sizes (0.25 mm) generally cause eventual failure due to clogging and therefore require periodic raking to remove solids. With buried systems the medium sands (0.35 to 0.5 mm) can result in long-term operation without raking or solids removal, providing the hydraulic loading rate is kept around 1.2 gal/d·ft 2 or less (<50 mm/d). The sand must be washed and free of fines (Crites and Tchobanoglous, 1998). Typical design criteria for ISFs are presented in Table 10.6. 10.5.1.2 Recirculating Gravel Filters The recirculating sand filter was developed by Michael Hines (Hines and Favreau, 1974). The modern recirculating filter uses fine gravel, as shown in Figure 10.4. TABLE 10.4 Types of Trickling Biofilter Media for Pretreatment of On-Site System Wastewater Granular Media Biofilters Organic Media Biofilters Synthetic Media Biofilters Activated carbon AIRR (alternating intermittent recirculating reactor) Ashco-A RSF III™ Crushed brick Envirofilter™ modular recirculating media filter Eparco Expanded aggregate Glass (crushed) Glass (sintered) Gravel (recirculating gravel filter [RGF]) Phosphex™ system RIGHT ® Sand Stratified sand Slag Zeolite Ecoflow ® ECO-PURE Peat Peat moss Puraflo ® peat Woodchip trickling Advantex Aerocell Bioclere Rubber (shredded tires) SCAT™ Septi Tech Waterloo Source: Leverenz, H. et al., Review of Technologies for the Onsite Treatment of Wastewater in California , Report No. 02-2, prepared for the California State Water Resources Control Board, Sacramento, CA, Department of Civil and Environmental Engineering, University of California, Davis, 2002. DK804X_C010.fm Page 502 Friday, July 1, 2005 4:52 PM © 2006 by Taylor & Francis Group, LLC [...]... Flushing valve Air coil system Valve box 0. 5- to 0.75-in rock Filter sand Air coil (if used) 0.375-in pea gravel 0. 5- to 0.75-in rock To drainfield or pump vault 4-in slotted PVC underdrain pipe 30-mil PVC liner (b) Typical cross-section FIGURE 10. 3 Schematic of an intermittent sand filter: (a) plan view, and (b) profile of a 2-ft-deep sand filter (Courtesy of Orenco Systems, Inc., Sutherlin, OR.) A recirculation... LLC DK804X_C 010. fm Page 516 Friday, July 1, 2005 4:52 PM 516 Natural Wastewater Treatment Systems Barrier material Soil cap Distribution laterals Absorption bed Clean drain rock Sand fill material Top soil Tilled top soil Permeable soil Water table or fractured bedrock FIGURE 10. 7 Schematic of a typical mound system 10. 7.4 IMPORTED FILL SYSTEMS Fill systems involve importing suitable off-site soils... Subsurface Flow Constructed Wetlands for Wastewater Treatment: A Technology Assessment, EPA 832-R-9 3-0 01, U.S Environmental Protection Agency, Washington, D.C Reed, S.C., Crites, R.W., and Middlebrooks, E.J (1995) Natural Systems for Waste Management and Treatment, 2nd ed., McGraw-Hill, New York Ronayne, M.A., Paeth, R.A., and Wilson, S.A (1984) Oregon Onsite Experimental Systems Program, Oregon Department... the ion exchange medium © 2006 by Taylor & Francis Group, LLC DK804X_C 010. fm Page 510 Friday, July 1, 2005 4:52 PM 510 Natural Wastewater Treatment Systems TABLE 10. 9 Design Criteria for Recirculating Gravel Filters Design Factor Unit Range Typical in 1–5 2.5 Filter Medium Effective size Depth in 18–36 24 U.C . 493 10 On-Site Wastewater Systems Effluent disposal options for on-site systems range from soil absorption in con- ventional gravity leachfields to water reuse after high-tech membrane treatment. Individual. treatment. Individual on-site systems are the most prevalent wastewater management sys- tems in the country. This chapter describes the various types of on-site wastewater systems, wastewater disposal. systems •Modified conventional on-site systems • Alternative on-site systems • On-site systems with additional treatment The most common on-site system is the conventional on-site system that consists of