Natural Wastewater Treatment Systems - Chapter 10 doc

con-of on-site management systems.10.1 TYPES OF ON-SITE SYSTEMS While many types of on-site systems exist, most involve some variation ofsubsurface disposal of septic tank efﬂuent.. The

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con-of on-site management systems.

10.1 TYPES OF ON-SITE SYSTEMS

While many types of on-site systems exist, most involve some variation ofsubsurface disposal of septic tank efﬂuent The four major categories of on-sitesystems 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 tankeffluent is sometimes needed, and intermittent and recirculating granular-mediumfilters are often the economical choice Where further nitrogen removal isrequired, 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 individualon-site systems are presented in Table 10.1

DK804X_C010.fm Page 493 Friday, July 1, 2005 4:52 PM

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494 Natural Wastewater Treatment Systems

10.2 EFFLUENT DISPOSAL AND REUSE OPTIONS

Alternative inﬁltration systems (presented in Table 10.2) have been developed toovercome restrictive conditions such as:

• Very rapidly permeable soils

• Very slowly permeable soils

• Shallow soil over bedrock

• Shallow groundwater

• Steep slopes

• Groundwater quality restrictions

• Limited spaceThe alternatives for reuse of on-site system effluent include drip irrigation, sprayirrigation, groundwater recharge, and toilet flushing Drip irrigation is becomingmore popular for water reuse and is described in this chapter Spray irrigation ismore suited to larger flows (commercial, industrial, and small community flows)and is described in detail in Chapter 8 Groundwater recharge, which is used inareas 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 involvesmultiple steps of identiﬁcation, reconnaissance, and assessment The processbegins with a thorough examination of the soil characteristics, which includepermeability, depth, texture, structure, and pore sizes The nature of the soil proﬁleand 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

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On-Site Wastewater Systems 495

slope, existing landscape and vegetation, and surface drainage features After apotential 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

Pressure-dosed:

Conventional trench To reach uphill ﬁelds

Drip application Following additional treatment of septic tank

efﬂuent; to optimize use of available land area

Alternative Systems

Mound Systems

Evapotranspiration systems Zero discharge

Constructed wetlands Requires a discharge or subsequent inﬁltration (see

Chapter 7 )

Reuse Systems

Other Systems

Surface water discharge Allowed in some states following added treatment DK804X_C010.fm Page 495 Friday, July 1, 2005 4:52 PM

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Moderately Rapid Very Slow Shallow Deep Shallow Deep 0–5% >5%

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10.3.1 P RELIMINARY S ITE E VALUATION

The initial step in conducting a preliminary site evaluation is to determine thecurrent and proposed land use, the expected ﬂow and characteristics of thewastewater, and to observe the site characteristics The next step is to gatherinformation 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 thephase 2 investigation, which can include identifying depth to groundwater duringthe wettest period of the year and permeability tests to determine water absorptionrates, can also be determined at this time A list of typical regulatory factors foron-site disposal is presented in Table 10.3

TABLE 10.3

Typical Regulatory Factors in On-Site Systems

Setback distances (horizontal, separation from wells,

springs, surface waters, escarpments, site boundaries,

Maximum hydraulic loading rates for leachﬁelds gal/ft 2 ·d 1.5

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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 thesoil Backhoe pits are useful to allow a detailed examination of the soil proﬁle forsoil 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 thesiting 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 todetermine the required size of the soil absorption area The allowable hydraulicloading rate for the soil absorption system is determined from a curve or tablethat relates allowable loading rates to the measured percolation rate A typicalcurve 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 Thebottom of the test hole is placed at the same depth as the proposed bottom of theabsorption area Prior to measuring the percolation rate, the hole should be soakedfor a period of 24 hr Tests and acceptable procedures used by local regulatoryagencies 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)

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Although used commonly, the percolation test results, because of the nature

of the test, are not related to the performance of the actual leachﬁelds Manyagencies and states are abandoning the test in favor of detailed soil proﬁleevaluations The percolation test is only useful in identifying soil permeabilitiesthat are very rapid or very slow Percolation tests should not be used as the solebasis 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 individualhouseholds or for sites where the hydraulic capacity is borderline within the localregulations, 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 absorptioncapacity 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 isthen calculated by measuring the amount of water that is pumped into the soilover 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 tothe environment Because the oxidized form of nitrogen, nitrate nitrogen, is a publichealth concern in drinking water supplies, the areal loading of nitrogen is important

1 Determine the wastewater loading rate The unit generation factor ismultiplied 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 60mg/L)

3 Calculate the nitrogen loading Multiply the nitrogen concentration bythe wastewater loading:

Nitrogen loading (lb/ac·d) = L× Nc× C × 10–6 (10.1)DK804X_C010.fm Page 499 Friday, July 1, 2005 4:52 PM

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500 Natural Wastewater Treatment Systemswhere

L = Wastewater loading (gal/ac·d)

The loadings of nitrate nitrogen to the groundwater are reduced by denitriﬁcation

in the soil column As indicated in Chapter 8, denitrification depends on thecarbon 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% Forheavier soils or where high groundwater or slowly permeable subsoils reduce therate of percolation, the denitrification fraction can be estimated at 25% Thepercolate nitrate concentration can be calculated from Equation 10.2:

where

Np = Nitrate nitrogen in the leachﬁeld percolate (mg/L)

Nc = 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 tankeffluent on an areal basis to 45 g/ac·d Determine the housing density withconventional septic tank effluent–soil absorption systems that will comply withthe ordinance Assume a total nitrogen content in the septic tank effluent of 60mg/L and a household wastewater generation of 175 gal/d

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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

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·ft2 (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

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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·ft2 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

Synthetic Media Biofilters

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.

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A recirculation tank is used to allow multiple passes of wastewater over the bed

A valve in the recirculation tank allows ﬁltered efﬂuent to be discharged

Recir-culating ﬁne gravel ﬁlters (RFGFs) use coarser media and higher hydraulic

loading rates than ISFs The performance of RFGFs is presented in Table 10.7

Recirculating gravel ﬁlters can nitrify effectively (over 90%) One consideration

in nitriﬁcation, particularly with ammonia levels that can exceed 60 mg/L, is

adequate alkalinity in the applied wastewater As ammonia is nitriﬁed, 7 mg of

alkalinity is destroyed for every 1 mg of ammonia oxidized to nitrate

Denitriﬁ-cation will recover a portion of the alkalinity, but lack of alkalinity in a soft,

low-alkalinity wastewater may cause the pH to drop, which will impact the ability to

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.)

Air coil system

Flushing valve Valve box

Air coil (if used)

To drainfield or pump vault 30-mil

PVC liner

4-in slotted PVC underdrain pipe

0.5- to 0.75-in rock

0.375-in pea gravel

Filter sand 0.5- to 0.75-in rock

0.75-in PVC lateral with 0.125-in.

orifices facing upward

Air coil (optional)

Flushing valve housing

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TABLE 10.5

Performance of Intermittent Sand Filters

Location (Ref.)

Effective Sand Size (mm)

Loading Rate (gal/ft 2 ·d)

BOD 5 Total Nitrogen Influent

(mg/L)

Effluent (mg/L)

Percent Removal (%)

Influent (mg/L)

Effluent (mg/L)

Stinson Beach, California (Nolte Associates, 1992a) 0.25–0.3 1.23 203 11 94 57 41 28

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completely nitrify the wastewater The design criteria for recirculating gravelﬁlters are presented Table 10.8

10.5.1.3 Septic Tank with Attached Growth Reactor

This system involves a small trickling filter unit placed above the septic tank.Septic tank effluent, which is pumped over the filter, is nitrified as it passes

a Based on peak ﬂow.

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through and over the plastic medium The system is shown schematically inFigure 10.5 A number of experimental units have been installed in septic tanks.The best performance with a plastic trickling ﬁlter medium has been achievedwith a hydraulic loading rate of 2.5 gal/min (9.5 L/min) over a unit 3 ft (0.9 m)deep containing hexagonally corrugated plastic with a surface area of 67 ft2/ft3

(226 m2/m3) A total nitrogen removal of 78% has been reported with an efﬂuentnitrogen concentration of less than 15 mg/L (Ball, 1995) The performance ofthese systems is summarized in Table 10.9 Recent studies have shown thevariability of performance (Loomis et al., 2004) Alternative ﬁlter media that have

FIGURE 10.4 Recirculating gravel ﬁlter.

SEPTIC TANK

RECIRCULATING FINE GRAVEL FILTER

FUTURE REPLACEMENT AREA

4-IN UNDERDRAIN LEADING TO A RECIRCULATING/MIXING TANK DRAINROCK

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been tested include the foam medium used in the Waterloo ﬁlter and the textilechips used in the textile bioreactor

10.5.1.4 RSF2 Systems

In the RSF2 system, a recirculating sand filter is used for nitrification and iscombined with an anaerobic filter for denitrification (Sandy et al., 1988) A flowdiagram for the RSF2 system is presented in Figure 10.6 Septic tank effluent isdischarged to one end of a rock storage filter, which is directly below and in thesame compartment as the RSF Septic tank effluent flows horizontally through the

Hydraulic Application Rate Field Capacity

Filled (%)b

(mm/dose) (gal/ft 2 ·dose)

a For 1 ft 2 of surface area and depth of 1.25 ft.

b Five% as volumetric water content (water volume/total volume) (Bouwer, 1978).

Source: Crites, R.W and Tchobanoglous, G., Small and Decentralized Wastewater ment Systems, McGraw-Hill, New York, 1998 With permission.

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Manage-TABLE 10.8

Performance of Recirculating Gravel Filters

Location (Ref.)

Effective Medium Size (mm)

Loading Rate (gal/ft 2 ·d)

BOD 5 Total Nitrogen Influent

(mg/L)

Effluent (mg/L)

Influent (mg/L)

Effluent (mg/L)

Source: Adapted from Reed et al (1995) and Leverenz et al (2002).

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rock and enters a pump chamber at the other end The septic tank effluent is pumpedover the RSF, where it is nitrified Filtrate is collected from near the top of therock storage filter, directed into a second pump chamber, and returned to theanaerobic environment of the septic tank, where raw wastewater can serve as acarbon source for denitrification A portion of effluent from the second pumpchamber is discharged for disposal Experiments with the RSF2 system producednitrogen removals of 80 to 90% Total nitrogen concentrations in the effluent rangedfrom 7.2 to 9.6 mg/L (Sandy et al., 1988) The rock storage zone, filled with 1.5-

in (38-mm) rock, was effective in promoting denitrification An alternative ification is to add the fixed medium (plastic, textile sheets) for biomass growth intothe recirculation tank Nitrified effluent from the recirculating sand filter is mixedwith the incoming septic tank effluent and flows past the attached biomass, whereany residual dissolved oxygen is consumed rapidly and the nitrate is denitrifiedusing the organic matter in the septic tank effluent as the carbon source

mod-10.5.1.5 Other Nitrogen Removal Methods

Other types of media have been used in bioreactors, including crushed glass,sintered glass, expanded aggregate, and crushed brick (Leverenz et al., 2002).The performance of three of these media ﬁlters is presented in Table 10.10 Othernitrogen methods that have been conceptualized include ammonia removal byion exchange and nitrogen removal by denitriﬁcation in soil trenches Attemptshave been made to remove ammonia by ion exchange using zeolite at Los Osos,California, and other locations (Nolte Associates, 1994) The attempts have beengenerally unsuccessful to date because of inadequate volumes of zeolite used andthe high cost of frequent regeneration or replacement of the ion exchange medium

FIGURE 10.5 Septic tank with attached-growth reactor for the removal of nitrogen.

(Courtesy of Orenco Systems, Inc., Sutherlin, OR.)

Spray nozzle

Trickling filter medium

Effluent

Dosing pump for trickling filter Effluent pump Influent

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TABLE 10.9 Design Criteria for Recirculating Gravel Filters

a Based on peak ﬂow.

FIGURE 10.6 Flow diagram for RSF2 system for the removal of nitrogen.

Wastewater

from

residence

Septic tank

Rock storage filter

Pump basin no.1

Pump basin no 2 Sand

filter To disposal

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Phosphorus removal is seldom required for on-site systems; however, when it isrequired, the soil mantle is the most cost-effective place to remove and retainphosphorus (see Chapter 8) Attempts to remove phosphorus in peat beds haveusually been unsuccessful unless iron or limestone is present or added to the bed

In Maryland, the use of iron ﬁlings plowed into the peat bed was successful inremoving phosphorus

10.6 DISPOSAL OF VARIOUSLY

TREATED EFFLUENTS IN SOILS

The disposal of partially treated wastewater into soils involves two major siderations: (1) treatment of the effluent so it does not contaminate surface orgroundwater, and (2) hydraulic flow of the effluent through the soil and awayfrom the site Pretreatment of the raw wastewater affects the degree of treatmentthat the soil–aquifer must achieve after the pretreated effluent is applied to thesoil absorption system Treatment of wastewater in soil has long been recognized(Crites et al., 2000) The soil is a combined biological, chemical, and physicalfilter Wastewater flowing through soil is purified of organic and biologicalconstituents, as described in Chapter 8 Septic tank effluent has sufficient solidsand organic matter to form a biological mat (“biomat”) in the subsurface,

con-TABLE 10.10

Performance Studies of Alternative Media

Parameter

Expanded Shalea Advantexb

Crushed Glassc

Efﬂuent total suspended solids e 5 (95) 3 (90) 2.5 (95)

a 24 in of LECA ® (light expanded clay aggregate) (Anderson et al., 1998).

b Roseburg, Oregon (Bounds et al., 2000).

c Oswego, New York (Elliott, 2001).

d In gal/ft 2 ·d.

e In mg/L (% removal).

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.

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particularly if gravity flow application is used More highly treated effluent andpressure-dosed application results in little, if any, biomat formation, and theflow through the soil is only inhibited by the hydraulic conductivity of the soil.Allowable hydraulic loading rates for variously treated effluents are presented

dis-10.7.1 G RAVITY L EACHFIELDS

Septic tank efﬂuent ﬂows by gravity into a series of trenches or beds for subsurfacedisposal Trenches are usually shallow, level excavations that range in depth from

1 to 5 ft (0.3 to 1.5 m) and in width from 1 to 3 ft (0.3 to 0.9 m) The bottom

of the trench is ﬁlled with 6 in (150 mm) of washed drain rock The 4-in mm) perforated distribution pipe is next placed in the center of the trench

(100-TABLE 10.11

Allowable Hydraulic Loading Rates for Variously Treated Effluent

Allowable Hydraulic Loading Rates

Mass Loading Rate (g/m 2 ·d) Type of Effluent (in./d) (gal/ft 2 ·d) (mm/d) BOD 5 TSS TKN

a Increased from Siegrist’s values for BOD (800 mg/L), TSS (300 mg/L), and TKN (80 mg/L) and lowered hydraulic loading rate from 4 mm/d to 3 mm/d.

Note: BOD5, biochemical oxygen demand; TSS, total suspended solids; TKN, total Kjeldahl nitrogen.

Source: Adapted from Siegrist, R.L., in Proceedings of the Fifth National Symposium on vidual and Small Community Sewage Systems, American Society of Agricultural Engineers,

Indi-Chicago, IL, December 14–15, 1987.

Tiêu đề	On-Site Wastewater Systems
Trường học	Taylor & Francis Group, LLC
Chuyên ngành	Environmental Engineering
Thể loại	Chương
Năm xuất bản	2006
Thành phố	New York

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