OnSite Wastewater Systems

Effluent disposal options for on-site systems range from soil absorption in conventional gravity leachfields to water reuse after high-tech membrane treatment. Individual on-site systems are the most prevalent wastewater management systems 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.

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, evapotranspiration 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.

10.1 TYPES OF ON-SITE SYSTEMS

Native soil backfill gm®

2-in. minimum rock over pipe

6-in. minimum rock under pipe'

36-in. max

.75- to 2.5-in.-diameter washed drainrock x.

Fabric or building paper

6 in. minimum mmm

-4-in. distribution pipe

12 in. minimum in

Side wall absorption area (both sides)

FIGURE 10.1 Typical cross-section through conventional soil absorption system.

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

• 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

TABLE 10.1

Types of On-Site Wastewater

Disposal/Reuse System

Conventional Systems

Gravity leachfields/conventional trench Gravity absorption beds

Modified Conventional Systems

Gravity leachfields: Deep trench Shallow trench Pressure-dosed:

Conventional trench Shallow trench Drip application

Alternative Systems

Sand-filled trenches At-grade systems Fill systems

Mound Systems

Evapotranspiration systems Evaporation ponds Constructed wetlands

Reuse Systems

Drip irrigation Spray irrigation Graywater reuse

Other Systems

Holding tanks Surface water discharge

Disposal/Reuse Systems

Remarks

Most common system

To get below restrictive layers Enhanced soil treatment

To reach uphill fields Uphill and shallow sites Following additional treatment of septic tank effluent; to optimize use of available land area

Added treatment

Less expensive than mounds

Import soil

Zero discharge See Chapter 4

Requires a discharge or subsequent infiltration (see Chapter 7)

Usually follows added treatment Requires disinfection

Seasonal use alternative

Allowed in some states following added treatment 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 10.2

Appropriate On-Site Disposal Methods To Overcome Site Constraints

Method

Soil Permeability Bedrock Groundwater

Moderately

Very Rapid Rapid Very Slow Shallow Deep Shallow Deep

Slope

Small >5% Lot Size

Trenches Beds Pits

Sand-lined trenches •

and beds Drained systems Evaporation ponds •

ETA beds

Spray irrigation •

Drip irrigation •

Note: The symbol • indicates appropriate system; ET, evapotranspiration; ETA, evapotranspiration-absorption.

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)

Maximum slope for on-site disposal field Soil characteristics:

(See Table 10.12)

25-30

Depth

Percolation rate

Minimum depth to groundwater Septic tank (minimum size) Maximum hydraulic loading rates for leachfields Maximum loading rates for sand filters ft min/in. ft gal gal/ft2'd gal/ft2'd

10.3.1 Preliminary Site Evaluation

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 permeability (general or qualitative)

• Site drainage

• Existence of streams, drainage courses, or wetlands

• Existing and proposed structures

• Vegetation and landscape

10.3.2 Applicable Regulations

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.

-Ryon's line used

---Ryon's line including all points

USPHS Study, troublefree system * USPHS Study, troubled system

-Ryon's line used

---Ryon's line including all points

USPHS Study, troublefree system * USPHS Study, troubled system

10 20 30 40 50 60 70 80 Time for water surface to fall 1 inch (minutes)

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.)

10 20 30 40 50 60 70 80 Time for water surface to fall 1 inch (minutes)

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.)

10.3.3 Detailed Site Assessment

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 regulatory 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 absorption 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.

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 Hydraulic Assimilative Capacity

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 Nitrogen Loading from Conventional Effluent Leachfields

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 x 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 x Nc x C x 10-6 (10.1)

where

L = Wastewater loading (gal/ac-d).

Nc = Nitrogen concentration (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 Cumulative Nitrogen Loadings

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 percolation 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:

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 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 = 45 g/ac-d x 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 x 8.34)(10-6) L = 0.099 lb/ac-d/(60 mg/L x 8.34 lb/gal)(10-6) L = 197.8 gal/ac-d

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:

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 Nitrogen Removal

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 intermittently 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 ammonia are removed by bacterial oxidation. Intermittent application and venting of

TABLE 10.4

Types of Trickling Biofilter Media for Pretreatment of On-Site System Wastewater

Granular Media Biofilters

Organic Media Biofilters

Synthetic Media Biofilters

Ecoflow® ECO-PURE Peat Peat moss Puraflo® peat Woodchip trickling

Advantex

Aerocell

Bioclere

Rubber (shredded tires) SCAT™ Septi Tech Waterloo

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

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.

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.

orifices facing upward

(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.)

(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 tank is used to allow multiple passes of wastewater over the bed. A valve in the recirculation tank allows filtered effluent to be discharged. Recirculating 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

TABLE 10.5

Performance of Intermittent Sand Filters

Location (Ref.)

Effective Sand Size (mm)

Loading Rate (gal/ft2d)

Florida (Grantham et al., 1949)

0.25-0.46

1.7—4.0

Florida (Furman et al., 1955)

0.25-1.04

2.0-13.0

Oregon (Ronayne et al., 1984)

0.14-0.3

0.33-0.88

Stinson Beach, California (Nolte Associates, 1992a)

0.25-0.3

1.23

University of California, Davis (Nor, 1991)

0.29-0.93

1.0^.0

Paradise, California (Nolte Associates, 1992a)

0.3-0.5

0.5

Placer County, California (Cagle and Johnson, 1994)

0.25-0.65

1.23

Gloucester, Maine (Jantrania et al., 1998)

0.8

Total Nitrogen

Percent

Influent Effluent Removal

148 14 90

217 3.2 98

203 11 94

148 6 96

Percent

Influent Effluent Removal

38 19 50

TABLE 10.6

Design Criteria for Intermittent Sand Filters Treating Septic Tank Effluent

Design Factor

Filter Medium

Material Effective size Uniformity coefficient Depth

Underdrain Bedding

Type

Size

Underdrain Piping

Type Size Slope

Pressure Distribution

Pipe size Orifice size Head on orifice Lateral spacing Orifice spacing Design Parameters Hydraulic loadinga BOD loading Dosing frequency Dosing tank volume Filter medium temperature a Based on peak flow.

Range

Gravel or stone 0.375-0.75

Slotted 3-4 0-1

Typical

Medium sand 0.35 3.5 24

Gravel 0.5

Perforated

completely nitrify the wastewater. The design criteria for recirculating gravel filters 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

System Schematic

PEA GRAVEL DISTRIBUTION PIPE

PEA GRAVEL DISTRIBUTION PIPE

RECIRCULATING/MIXING TANK Typical Cross-Section Plan View

FIGURE 10.4 Recirculating gravel filter.

RECIRCULATING/MIXING TANK Typical Cross-Section Plan View

FIGURE 10.4 Recirculating gravel filter.

through and over the plastic medium. The system is shown schematically in Figure 10.5. A number of experimental units have been installed in septic tanks. The best performance with a plastic trickling filter medium has been achieved with 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 effluent nitrogen concentration of less than 15 mg/L (Ball, 1995). The performance of these systems is summarized in Table 10.9. Recent studies have shown the variability of performance (Loomis et al., 2004). Alternative filter media that have

TABLE 10.7

Analysis of Volume per Dose for Various Hydraulic Loading Rates and Dosing Frequencies for Intermittent Sand Filters3

Hydraulic Loading Rate (gal/ft2d)

TABLE 10.7

Analysis of Volume per Dose for Various Hydraulic Loading Rates and Dosing Frequencies for Intermittent Sand Filters3

Hydraulic Loading Rate (gal/ft2d)

Dosing Frequency (times/d)

Hydraulic Application Rate

Field Capacity Filled

(mm/dose)

(gal/ft2dose)

(%)b

1

40

1

217

2

20

0.S

107

4

10

0.2S

S3

8

S

0.12

26

12

3.3

0.083

18

24

1.67

0.042

9.0

1

81

2

427

2

40

1

217

4

20

0.S

107

8

10

0.2S

S3

12

6.7S

0.12

26

24

3.38

0.083

18

1

163

4

8SS

2

82

2

427

4

41

1

217

8

20

0.S

107

12

14

0.33

71

24

6.79

0.17

36

a For 1 ft2 of surface area and depth of 1.2S ft.

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

a For 1 ft2 of surface area and depth of 1.2S ft.

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

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

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

10.5.1.4 RSF2 Systems

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

TABLE 10.8

Performance of Recirculating Gravel Filters

Effective

Loading

Medium Size

Rate

Location (Ref.)

(mm)

(gal/fP-d)

Michigan (Loudon et al., 1984)

0.3

3.0

Oregon (Ronayne et al., 1984)

1.2

1.45

Paradise, California (Nolte Associates, 1992)

3.0

4.4

Paradise, California (Nolte Associates, 1992)

3.0

2.5

Martinez, California (Crites et al., 1997)

3.0

3.0

Minnesota (Christopherson et al., 2001)

-

5.0

Gloucester, Massachusetts (Jantrania et al., 1998)

3.0

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

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

BOD5

Total Nitrogen

Percent

Influent Effluent Removal

240 25 90

217 2.7 99

134 12 91

60 8 87

Percent

Influent Effluent Removal

92 34 60

58 32 45

63 35 44

57 26 54

Spray nozzle

Spray nozzle

FIGURE 10.5 Septic tank with attached-growth reactor for the removal of nitrogen. (Courtesy of Orenco Systems, Inc., Sutherlin, OR.)

rock and enters a pump chamber at the other end. The septic tank effluent is pumped over the RSF, where it is nitrified. Filtrate is collected from near the top of the rock storage filter, directed into a second pump chamber, and returned to the anaerobic environment of the septic tank, where raw wastewater can serve as a carbon source for denitrification. A portion of effluent from the second pump chamber is discharged for disposal. Experiments with the RSF2 system produced nitrogen removals of 80 to 90%. Total nitrogen concentrations in the effluent ranged from 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 modification is to add the fixed medium (plastic, textile sheets) for biomass growth into the recirculation tank. Nitrified effluent from the recirculating sand filter is mixed with the incoming septic tank effluent and flows past the attached biomass, where any residual dissolved oxygen is consumed rapidly and the nitrate is denitrified using the organic matter in the septic tank effluent as the carbon source.

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. Other nitrogen methods that have been conceptualized include ammonia removal by ion exchange and nitrogen removal by denitrification in soil trenches. Attempts have been made to remove ammonia by ion exchange using zeolite at Los Osos, California, and other locations (Nolte Associates, 1994). The attempts have been generally unsuccessful to date because of inadequate volumes of zeolite used and the high cost of frequent regeneration or replacement of the ion exchange medium.

TABLE 10.9

Design Criteria for Recirculating Gravel Filters

TABLE 10.9

Design Criteria for Recirculating Gravel Filters

Design Factor

Unit

Range

Typical

Filter Medium

Effective size

in.

1-5

2.5

Depth

in.

18-36

24

Uniformity coefficient

U.C.

<2.5

2.0

Underdrains

Size

in.

3-4

4

Slope

%

0-0.1

0

Pressure Distribution

Pipe size

in.

1-2

1.5

Orifice size

in.

1/8-1/4

1/8

Head on orifice

ft

3-6

5

Lateral spacing

ft

1.5-4

2

Orifice spacing

ft

1.5-4

2

Design Parameters

Hydraulic loading"

gal/ft2'd

3-5

4

BOD loading

lb/ft2-d

0.002-0.008

<0.005

Recirculation ratio

Unitless

3:1-5:1

4:1

Dosing Times

Time on

min

<2-3

<2-2

Time off

min

15-25

20

Dosing

Frequency

times/d

48-120

Dosing tank volume

flow/d

0.5-1.5

1

a Based on peak flow.

a Based on peak flow.

Pump Pump

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

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

TABLE 10.10

Performance Studies of Alternative Media

Parameter

Expanded Shalea

Glassc

Hydraulic loading rated

1.35

1.8

Effluent BODe

1 (99)

5 (98)

10.7 (94)

Effluent total suspended solidse

5 (95)

3 (90)

2.5 (95)

Effluent nitrogene

29 (39)

7 (78)

19.7 (55)

Effluent phosphoruse

0.5 (94)

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/ft2-d. e In mg/L (% removal).

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/ft2-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.

10.5.2 Phosphorus Removal

Phosphorus removal is seldom required for on-site systems; however, when it is required, the soil mantle is the most cost-effective place to remove and retain phosphorus (see Chapter 8). Attempts to remove phosphorus in peat beds have usually 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 in removing phosphorus.

10.6 DISPOSAL OF VARIOUSLY

TREATED EFFLUENTS IN SOILS

The disposal of partially treated wastewater into soils involves two major considerations: (1) treatment of the effluent so it does not contaminate surface or groundwater, and (2) hydraulic flow of the effluent through the soil and away from the site. Pretreatment of the raw wastewater affects the degree of treatment that the soil-aquifer must achieve after the pretreated effluent is applied to the soil absorption system. Treatment of wastewater in soil has long been recognized (Crites et al., 2000). The soil is a combined biological, chemical, and physical filter. Wastewater flowing through soil is purified of organic and biological constituents, as described in Chapter 8. Septic tank effluent has sufficient solids and organic matter to form a biological mat ("biomat") in the subsurface,

TABLE 10.11

Allowable Hydraulic Loading Rates for Variously Treated Effluent

Allowable Hydraulic Mass Loading Rate

Loading Rates (g/m2d)

TABLE 10.11

Allowable Hydraulic Loading Rates for Variously Treated Effluent

Allowable Hydraulic Mass Loading Rate

Loading Rates (g/m2d)

Type of Effluent

(in./d)

(gal/ft2d)

(mm/d)

BOD5

TSS

TKN

Restaurant septic tanka

0.12

0.07

3

2.4

0.9

0.24

Domestic septic tank

0.4

0.25

10

1.5

0.8

0.55

Graywater septic tank

0.6

0.37

15

1.8

0.6

0.22

Domestic aerobic unit

0.8

0.50

20

0.7

0.8

0.30

Domestic sand filter

3.0

1.87

76

0.3

0.75

0.75

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.

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 Individual and Small Community Sewage Systems, American Society of Agricultural Engineers, Chicago, IL, December 14-15, 1987.

particularly if gravity flow application is used. More highly treated effluent and pressure-dosed application results in little, if any, biomat formation, and the flow through the soil is only inhibited by the hydraulic conductivity of the soil. Allowable hydraulic loading rates for variously treated effluents are presented in Table 10.11.

10.7 DESIGN CRITERIA FOR ON-SITE DISPOSAL ALTERNATIVES

Gravity-flow leachfields are the most common type of on-site wastewater disposal. This type of on-site disposal functions well for sites with deep, relatively permeable soils, where groundwater is deep and the site is relatively level.

10.7.1 Gravity Leachfields

Septic tank effluent flows by gravity into a series of trenches or beds for subsurface disposal. 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. (100mm) perforated distribution pipe is next placed in the center of the trench.

Additional drain rock is placed over the top of the distribution pipe, followed by a layer of barrier material, typically building paper or fabric. The purpose of the barrier material is to prevent migration of fines from the backfill into the drain rock and avoid clogging of the drain rock by the clay or silt particles. The infiltrative surfaces in a leachfield trench are the bottom and the sidewalls; however, as a clogging layer of biological solids or "biomat" develops, the infiltration through the bottom of the trench decreases and the sidewalls become effective and become the long-term route for water passage.

Bed systems consist of an excavated area or bed with perforated distribution pipes that are 3 to 6 ft (0.9 to 1.8 m) apart. The route for water passage out of the bed is through the bottom. Bed systems can also use infiltration chambers, which create underground caverns over the soil's infiltrative surface and therefore do not need the gravel or barrier material.

Leaching chambers constructed out of concrete are open-bottomed shells that replace perforated pipe and gravel for distribution and storage of the wastewater. The chambers interlock to form an underground cavern over the soil. Wastewater is discharged into the cavern through a central weir, trough, or splash plate and allowed to flow over the infiltrative surface in any direction. Access holes in the top of the chambers allow the surface to be inspected and maintained as necessary. Many leaching chamber systems have been installed in the northeastern United States.

Typical criteria for siting of leachfield systems are presented in Table 10.12. Loading rates for trench and bed systems can be based on percolation test results and regulatory tables, on soil characteristics, or a combination of both. Disposal field loading rates recommended by the USEPA for design, based on bottom area, for various types of soils and observed percolation rates are shown in Table 10.13.

The loading rate based on the most conservative criterion is to assume that the percolation rate through the soil will eventually be reduced to coincide with the percolation rate through the biomat. On this basis, the hydraulic loading rate is 0.125 gal/ft2-d (5 L/m2-d) based on trench sidewall area only (Winneberger, 1984).

Where the site soils contain significant amounts of clay, it is suggested that the disposal field be divided into two fields and that the two fields be used alternately every 6 months. When two fields are used, the actual hydraulic loading rate for the field in operation is 0.25 gal/ft2-d (10 L/m2-d).

10.7.2 Shallow Gravity Distribution

Shallow leachfields offer the benefits of lower cost and higher biological treatment potential because the upper soil layers have the most bacteria and fungi for wastewater renovation (Reed and Crites, 1984). The State of Oregon recently allowed the use of leachfield trenches without gravel that are 10 in. (250 mm) deep and 12 in. (300 mm) wide (Ball, 1994).

TABLE 10.12

Design Considerations in Siting Leachfields

Item

Landscape Forma

Slope3

Typical Horizontal Setbacks'5

Water supply sells Surface waters, springs Escarpments, man-made cuts Boundary of property Building foundations

Soil

Unsaturated depth

Texture

Structure Color

Layering

Swelling clays

Criteria

Level, well-drained areas; crests of slopes; convex slopes are most desirable. Avoid depressions, bases of slopes, and concave slopes unless suitable surface drainage is provided.

0-25%; slopes in excess of 25% can be used, but construction equipment selection is limited.

2-4 ft (0.6-1.2 m) of unsaturated soil should exist between the bottom of the disposal field and the seasonally high water table or bedrock.

Soils with sandy or loamy textures are best suited; gravelly and cobbley soils with open pores and slowly permeable clay soils are less desirable.

Strong granular, blocky, or prismatic structures are desirable; platey or unstructured massive soils should be avoided.

Bright, uniform colors indicate well-drained, well-aerated soils; dull, gray, or mottled soils indicate continuous or seasonal saturation and are unsuitable.

Soils exhibiting layers with distinct textural or structural changes should be evaluated carefully to ensure that water movement will not be severely restricted.

Presence of swelling clays requires special consideration in construction; location may be unsuitable if extensive.

a Landscape position and slope are more restrictive for seepage beds because of the depth of cut on the upslope side.

b Intended only as a guide. Safe distance varies from site to site, based on local codes, topography, soil permeability, groundwater gradients, geology, etc.

Source: Adapted from USEPA, Design Manual: Onsite Wastewater Treatment and Disposal Systems, Municipal Environmental Research Laboratory, U.S. Environmental Protection Agency, Cincinnati, OH, 1980.

TABLE 10.13

Recommended Rates of Wastewater Application for Trench and Bed Bottom Areas

Percolation Rate Application Rate

TABLE 10.13

Recommended Rates of Wastewater Application for Trench and Bed Bottom Areas

Percolation Rate Application Rate

Soil Texture

(min/in.)

(gal/ft2-d)ab

Gravel, coarse sand

<1

Not suitable0

Coarse to medium sand

1-5

1.2

Fine sand, loamy sand

6-15

0.8

Sand loam, loam

16-30

0.6

Loam, porous silt loam

31-60

0.45

Silty clay loam, clay loamd,e

61-120

0.2

Clays, colloidal clays

>120

Not suitablef

a Rates based on septic tank effluent from a domestic waste source. A safety factor may be desirable for wastewaters of significantly different strength or character. b May be suitable for sidewall infiltration rates.

c Soils with percolation rates <1 min/in. may be suitable for septic tank effluent if a 2-ft layer of loamy sand or other suitable soil is placed above or in place of the native topsoil. d These soils are suitable if they are without significant amounts of expandable clays. e Soil is easily damaged during construction.

f Alternative pretreatment may be required, as well as alternative disposal (wetlands or evapotranspiration systems).

Source: Adapted from USEPA, Design Manual: Onsite Wastewater Treatment and Disposal Systems, Municipal Environmental Research Laboratory, U.S. Environmental Protection Agency, Cincinnati, OH, 1980.

10.7.3 Pressure-Dosed Distribution

Pressure dosing can be achieved using either a dosing siphon or a pump. A pressure distribution system has the advantages over gravity distribution of providing a uniform dose to the entire absorption area, promoting unsaturated flow, and providing a consistent drying and reaeration period between doses. Pressure-dosed distribution can allow the absorption site to be at a higher elevation from the septic tank and will also allow a shallow (6- to 12-in.) distribution network. With screened septic tank effluent or sand filter effluent, the distribution system can use 0.125-in. (3-mm) orifices, typically spaced 2 to 4 ft (0.6 to 1.2 m) apart. For septic tank effluent, the orifice size is typically 0.25 in. (6 mm). The spacing and sizing of orifices should be uniform because the objective of pressure dosing is to provide uniform distribution with unsaturated flow beneath the pipe. In heavier soils, the spacing can be increased to 4 to 6 ft (1.2 to 1.8 m).

Water table or fractured bedrock

FIGURE 10.7 Schematic of a typical mound system.

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 and placing them over the soil absorption area to overcome limited depth of soil or limited depth to ground-water. Care must be taken when selecting suitable soil to use in a fill system and in the timing and conditions of importing the soil. Several conditions must be satisfied to construct a successful fill system:

• Native soil should be scarified prior to import of fill.

• The fill should be placed when the soil is dry.

• The fill material should also be dry to prevent compaction.

• The first 6 in. (150 mm) of fill should be mixed thoroughly with the native soil.

10.7.5 At-Grade Systems

The concept of the at-grade system was developed in Wisconsin as an intermediate system between conventional in-ground distribution and the mound system. The aggregate or drain rock is placed on the soil surface (at-grade) and a soil cap is added over the top. Typically, the area for the at-grade system is tilled, the drain rock is placed on the tilled area, the distribution pipe is positioned within the drain rock, synthetic fabric is spread over the drain rock, and final soil cover (12 in. or 300 mm) is placed over the system. At-grade systems do not require the 24 in. (600 mm) of sand that mounds have and, therefore, are less expensive.

10.7.6 Mound Systems

Mound systems are, in effect, bottomless intermittent sand filters. Components of a typical mound, as shown in Figure 10.7, include a 24-in. (600-mm) layer of sand, clean drain rock, distribution laterals, barrier material, and the soil cap. Mounds are pressure dosed, usually 4 to 6 times per day. Mounds were first developed by the North Dakota Agricultural College in the late 1940s. They were known as NODAK systems and were designed to overcome problems with slowly

TABLE 10.14

Infiltration Rates for Determining Base Area of Mound

Native On-Site Soil

Percolation Rate (min/in.)

Infiltration Rate

(gal/ft2d)

Sand, sandy loam Loam, silt loams

31-45

0-30

0.75

Silt loams, silty clay loams Clay loams, clay

61-120

46-60

0.50

0.25

Source: Adapted from USEPA, Design Manual: Onsite Wastewater Treatment and Disposal Systems, Municipal Environmental Research Laboratory, U.S. Environmental Protection Agency, Cincinnati, OH, 1980.

permeable soils and areas that had high groundwater tables (Ingham, 1980; WPCF, 1990). Mounds may be used on sites that have slopes up to 12%, provided the soils are permeable. If the native soils are slowly permeable, the use of mounds should be restricted to slopes of less than 6%. The design of mound systems is a two-step process. Percolation tests are conducted on the native soils on the site at the depth at which the mound base will exist. The values of the measured percolation rate are correlated to the design infiltration rate in Table 10.14, and the infiltration rate is then used to calculate the base area of the mound. The second step is to design the mound section. On the basis of the type of material used to construct the mound, the area of the application bed in the mound is determined. Mound fill materials are listed in Table 10.15 along with the corresponding design infiltration rate for determining the bed area (Otis, 1982).

10.7.7 Artificially Drained Systems

Sometimes a high-groundwater condition can be overcome by draining the groundwater away from the site. High groundwater tables in the area of the soil absorption fields may be artificially lowered by vertical drains or underdrains. Underdrains can be perimeter drains, used for level sites and sites up to 12% in slope, or curtain drains (upslope side only), for sites with slopes greater than 12% (Nolte Associates, 1992b).

10.7.8 Constructed Wetlands

Constructed wetlands can be used for on-site treatment as well as on-site disposal and reuse. As described in Chapter 6, constructed wetlands can be either the free water surface type or the subsurface flow type. For on-site systems in close proximity to children, the subsurface flow wetlands are most appropriate. A large number of subsurface wetlands have been constructed and placed in operation in Louisiana, Arkansas, Kentucky, Mississippi, Tennessee, Colorado, and New

TABLE 10.15

Mound Fill Materials and Infiltration Rates

Characteristics (% by weight)

Infiltration Rate

Material

(gal/ft2d)

Medium sand

>25%, 0.25-0.2 mm <30-35%, 0.05-0.25 mm <5-10%, 0.002-0.05 mm 5-15% clay 88-93% sand

Sandy loam Sand/sandy loam

Source: Adapted from USEPA, Design Manual: Onsite Wastewater Treatment and Disposal Systems, Municipal Environmental Research Laboratory, U.S. Environmental Protection Agency, Cincinnati, OH,

1980.

Mexico. These systems serve single-family dwellings, public facilities and parks, apartments, and commercial developments (Reed, 1993). On-site wetlands are SSF wetlands and are described in Chapter 7.

10.7.9 Evapotranspiration Systems

In arid climates, evapotranspiration (ET) systems can be used for effluent disposal. Effluent from the septic tank is applied through perforated pipes to a sand bed underlain by a liner. The sand depth is typically 24 to 30 in. (0.6 to 0.75 m). Bernhart (1973) recommended a sand depth of 18 in. (0.45 m). The surface of the sand bed is covered with a shallow layer of topsoil, which can be planted to water-tolerant vegetation. Treated wastewater is drawn up through the sand by capillary forces and by the plant roots, and it is evaporated or transpired to the atmosphere. A fine sand (0.1 mm) is recommended to maximize the capillary rise. Observation wells are used to monitor the depth of water in the sand beds.

The ET system can also be designed without a liner, and the resultant system is referred to as an evapotranspiration-absorption (ETA) system. The ETA approach can be used where percolation is acceptable and possible. An ETA system is similar to an at-grade system, except for the addition of surface vegetation. Both ET and ETA systems are designed using the hydraulic loading rate. For ET systems, the hydraulic loading rate is the minimum monthly net evapotranspiration rate for at least 10 years of record. For ETA systems, the minimum monthly percolation rate is added to the minimum ET rate to determine the design hydraulic loading rate. The bed area for ET and ETA systems can be determined using Equation 10.3:

where:

A =

Bed area (ft2).

Q =

Annual flow (ft3/yr).

ET =

Annual potential evapotranspiration rate (ft/yr)

Pr =

Annual precipitation rate (ft/yr).

P=

Annual percolation rate (ft/yr).

For ET systems, the percolation rate is zero; for ETA systems, the percolation rate should be determined based on long-term saturated flow conditions.

Example 10.2. Design of an Evapotranspiration System Design an evapotranspiration system for a cluster of homes with a design flow of 1800 gal/d. The annual lake evaporation rate is 50 in./yr, and the precipitation rate for the wettest year in 10 is 20 in./yr.

Solution

1. Convert the daily flow to an annual flow:

Q = 1800 gal/d x 365 d/yr = 657,000 gal/yr x 1/7.48 gal/ft3 = 87,834 ft3/yr

2. Calculate the hydraulic loading rate:

Comment

A factor of safety, typically 15 to 20%, should be added to the bed area to account for variations in precipitation and flow rate.

10.8 design criteria for

ON-SITE REUSE ALTERNATIVES

Reuse alternatives for on-site systems include drip irrigation and spray irrigation. 10.8.1 Drip Irrigation

Drip irrigation technology has advanced over the years to where non-clog emitters are available for both surface and subsurface uses. Sand filter and other high-quality effluent can be used in drip irrigation of landscape and other crops. Periodic chlorination of the drip tubing has been found to be necessary to avoid clogging growths in the distribution lines and emitters. Modern drip emitters have been designed not to be clogged by roots. For example, the Geoflow™ emitter has been treated with a herbicide to protect it from root intrusion. The emitters are designed with a turbulent flow path to minimize clogging from suspended solids. These emitters operate at a flow rate of 1 to 2 gal/hr with 0.06-to 0.07-in. (1.5- to 1.8-mm)-diameter openings. The drip irrigation system usually requires 15 to 25 lb/in2 pressure. It may be necessary to flush the lines and to apply periodic doses of chlorine for control of clogging from bacterial growth. A typical on-site drip irrigation system consists of emitter lines placed on 2-ft (0.6-m) centers with a 2-ft (0.6-m) emitter spacing. This spacing is typical for sandy and loamy soils. Closer spacings of 15 to 18 in. (0.4 to 0.45 m) are used on clay soils where lateral movement of water is restricted. The emitter lines are placed at depths of 6 to 10 in. (150 to 250 mm). Drip systems can be optimized to minimize nitrate movement through the soil. Nitrification of septic tank effluent without denitrification can increase nitrate movement. Short daily pulses increase nitrogen removal compared to continuous applications (Beggs et al., 2004).

Example 10.3. Design of a Drip Irrigation System

Design a drip irrigation system for the reuse of 300 gal/d of treated effluent. Use a design infiltration rate of 0.25 gal/ft2-d.

Solution

1. Determine the area needed for irrigation.

2. Lay out the 1200 ft2 as a 40-ft by 30-ft rectangle.

3. Select a spacing of the drip emitter lines of 2 ft. Use 20 emitter lines that are 30 ft long.

4. Use 1-gal/hr emitters, spaced at 2-ft intervals. Calculate the number of emitters.

30 ft per line - 2-ft spacing = 15 emitters per line 20 lines x 15 emitters per line = 300 emitters

5. Calculate the flow discharged from 300 emitters.

6. Calculate the time of operation per day.

7. Select a pump for the application. The pump must be able to supply 300 gal/d - (1.0 hr x 60 min/hr) = 5.0 gal/min at a pressure of 20 lb/ft2.

Comment

The emitters should be buried at a depth of 10 in.

10.8.2 Spray Irrigation

The use of spray irrigation for on-site disposal is relatively limited except in areas where housing density is low and other less expensive alternatives are not appropriate. Flows need to exceed 3 to 5 gal/min (11 to 19 L/min) to operate most single sprinklers. This relatively high flow generally means that spray irrigation is better suited to flows from an industrial, commercial, or institutional facility. In addition, for residential on-site systems, the additional treatment may need to include sand filtration and disinfection. The details of spray irrigation site assessment and design are presented in Chapter 8 in the discussion of slow-rate land treatment.

10.8.3 Graywater Systems

In older homes and in areas where water conservation or reuse is practiced because of water shortages or lack of wastewater disposal capacity, the laundry water and other non-toilet wastewater is often reused or disposed of separately from the "black" water that goes into the septic tank. The graywater includes organics, nutrients, and pathogens; however, it is perceived as being a benign source of wastewater that can be reused directly for landscape irrigation. Local health departments have allowed graywater reuse in rural areas but have often denied graywater reuse in urban areas. In California, regulations specify safe and acceptable methods of on-site reuse of graywater (California Resources Agency, 1994). California's graywater standards are now part of the state plumbing code, making it legal to use graywater everywhere in California.

10.9 CORRECTION OF FAILED SYSTEMS

The failure of subsurface on-site disposal systems is defined as the inability of the system to accept and absorb the design flow of effluent at the expected rate. When failure occurs soon after the system is put into operation, the failure may be the result of poor construction (Winneberger, 1987), po

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