Natural Wastewater Treatment Systems - Chapter 2 potx

Discharging systems would include those with asurface water discharge, such as treatment ponds, constructed wetlands, andoverland flow land treatment.. Underdrained slow rate or soil aqui

Assessment, and Site Selection

When conducting a wastewater treatment and reuse/disposal planning study, it isimportant to evaluate as many alternatives as possible to ensure that the mostcost-effective and appropriate system is selected For new or unsewered commu-nities, decentralized options should also be included in the mix of alternatives(Crites and Tchobanoglous, 1998) The feasibility of the natural treatment pro-cesses that are described in this book depends significantly on site conditions,climate, regulatory requirements, and related factors It is neither practical noreconomical, however, to conduct extensive field investigations for every process,

at every potential site, during planning This chapter provides a sequentialapproach that first determines potential feasibility and the necessary land require-ments and site conditions of each alternative The second step evaluates each sitecoupled with a natural treatment process based on technical and economic factorsand selects one or more for detailed investigation The final step involves detailedfield investigations (as necessary), identification of the most cost-effective alter-native, and development of the criteria necessary for the final design

2.1 CONCEPT EVALUATION

One way of categorizing the natural systems is to divide them between dischargingand nondischarging systems Discharging systems would include those with asurface water discharge, such as treatment ponds, constructed wetlands, andoverland flow land treatment Underdrained slow rate or soil aquifer treatment(SAT) systems may also have a surface water discharge that would be permittedunder the National Pollutant Discharge Elimination System (NPDES) Nondis-charging systems would include slow rate land treatment and SAT, onsite meth-ods, and biosolids treatment and reuse methods Site topography, soils, geology,and groundwater conditions are important factors for the construction of discharg-ing systems but are often critical components of the treatment process itself fornondischarging systems Design features and performance expectations for bothtypes of systems are presented in Table 2.1, Table 2.2, and Table 2.3 Special siterequirements are summarized in Table 2.1 and Table 2.2 for each type of systemfor planning purposes It is presumed that the percolate from a nondischargingsystem mingles with any groundwater that may be present The typical regulatory

12 Natural Wastewater Treatment Systems

requirement for compliance is the quality measured in the percolate/groundwater

as it reaches the project boundary

As noted in Table 2.1, SR and SAT systems can include surface dischargefrom underdrains, recovery wells, or cutoff ditches For example, the large SRsystem at Muskegon County, Michigan, has underdrains with a surface waterdischarge For the forested SR system at Clayton County, Georgia, the subflowfrom the wastewater application leaves the site and enters the local streams.Although the subflow does emerge in surface streams, which are part of thecommunity’s drinking water supplies, the land treatment system is not considered

to be a discharging system as defined by the U.S Environmental ProtectionAgency (EPA) and the State of Georgia

2.1.1 I NFORMATION N EEDS AND S OURCES

A preliminary determination of process feasibility and identification of potentialsites are based on the analysis of maps and other information The requirementsshown in Table 2.1 and Table 2.2, along with an estimate of the land area requiredfor each of the methods, are considered during this procedure The sources ofinformation and type of information needed are summarized in Table 2.3

TABLE 2.1 Special Site Requirements for Discharge Systems

Treatment ponds Proximity to a surface water for discharge, impermeable

soils or liner to minimize percolation, no steep slopes, out

of flood plain, no bedrock or groundwater within excavation depth

Constructed wetlands Proximity to a surface water for discharge, impermeable

soils or liner to minimize percolation, slopes 0–6%, out of flood plain, no bedrock or groundwater within excavation depth

Overland flow (OF) Relatively impermeable soils, clay and clay loams, slopes

0–12%, depth to groundwater and bedrock not critical but 0.5–1 m desirable, must have access to surface water for discharge or point of water reuse

Underdrained slow rate (SR) and soil aquifer treatment (SAT)

For SR, same as tables in Chapter 1 and Table 2.2 except for impermeable layer or high groundwater that requires the use of underdrains to remove percolating water; for SAT, wells or underdrains may remove percolating water for discharge

Planning, Feasibility Assessment, and Site Selection 13

TABLE 2.2

Special Site Requirements for Nondischarging System

Wastewater Systems

Slow rate (SR) Sandy loams to clay loams: >0.15 to <15 cm/hr permeability

preferred, bedrock and groundwater >1.5 m, slopes <20%, agricultural sites <12%

Soil aquifer treatment (SAT) or

rapid infiltration (RI)

Sands to sandy loams: 5 to 50 cm/hr permeability, bedrock and groundwater >5 m preferred, >3 m necessary, slopes

<10%; sites with slopes that require significant backfill for basin construction should be avoided; preferred sites are near surface waters where subsurface flow may discharge over non-drinking-water aquifers

Reuse wetlands Slowly permeable soils, slopes 0 to 6%, out of flood plain,

no bedrock or groundwater within excavation depth

Biosolids Systems

Land application Generally the same as for agricultural or forested SR systems Composting, freezing,

vermistabilization, or reed beds

Usually sited on the same site as the wastewater treatment plant; all three require impermeable barriers to protect groundwater; freezing and reed beds also require underdrains for the percolate

TABLE 2.3

Sources of Site Planning Information

Topographic maps Elevations, slope, water and drainage

features, building and road locations Natural Resources Conservation Service soil

14 Natural Wastewater Treatment Systems

2.1.2 L AND A REA R EQUIRED

The land area estimates derived in this section are used with the information inTable 2.1 and Table 2.2 to determine, with a study of the maps, whether suitablesites exist for the process under consideration These preliminary area estimatesare very conservative and are intended only for this preliminary evaluation Theseestimates should not be used for the final design

2.1.2.1 Treatment Ponds

The types of treatment ponds (described in Chapter 4) include oxidation ponds,facultative ponds, controlled-discharge ponds, partial-mix aerated ponds, com-plete-mix ponds, proprietary approaches, and modifications to conventionalapproaches The area estimate for pond systems will depend on the effluent qualityrequired (as defined by biochemical oxygen demand [BOD] and total suspendedsolids [TSS]), on the type of pond system proposed, and on the climate in theparticular geographic location A facultative pond in the southern United Stateswill require less area than the same process in Canada The equations given beloware for total project area and include an allowance for roads, levees, and unusableportions of the site

Oxidation Ponds

The area for an aerobic pond assumes a depth of 3 ft (1 m), a warm climate, a30-day detention time, an organic loading rate of 80 lb/ac·d (90 kg/ha·d), and aneffluent quality of 30 mg/L BOD and >30 mg/L TSS The planning area required

is calculated using Equation 2.1:

where

A pm = Total project area, (ac; ha)

k = Factor (3.0 × 10–5, U.S units; 3.2 × 10–3, metric)

Q = Design flow (gal/d; m3/d)

Facultative Ponds in Cold Climates

The area calculation in Equation 2.2 assumes an 80-day detention time, a pond

5 ft (1.5 m) deep, an organic loading of 15 lb/ac·d (16.8 kg/ha·d), an effluentBOD of 30 mg/L, and TSS > 30 mg/L The area required is:

where

A fc = Facultative pond site area (ac; ha)

k = Factor (1.6 × 10–4, U.S units; 1.68 × 10–2, metric)

Q = Design flow (gal/d; m3/d)

Planning, Feasibility Assessment, and Site Selection 15

Facultative Ponds in Warm Climates

Assume more than 60 days of detention in a pond 5 ft (1.5 m) deep and an organicloading of 50 lb/ac·d (56 kg/ha·d); the expected effluent quality is BOD = 30mg/L and TSS > 30 mg/L The area required is:

where

A fw = Facultative pond site area, warm climate (ac; ha)

k = Factor (4.8 × 10–5, U.S units; 5.0 × 10–3, metric)

Q = Design flow (gal/d; m3/d)

Controlled-Discharge Ponds

Controlled-discharge ponds are used in northern climates to avoid winter charges and in warm climates to match effluent quality to acceptable stream flowconditions The typical depth is 5 ft (1.5 m), maximum detention time is 180days, and the expected effluent quality is BOD < 30 mg/L and TSS < 30 mg/L.The required site area is:

where

A cd = Controlled-discharge pond site area (ac; ha)

k = Factor (1.32 × 10–4, U.S units; 1.63 × 10–2, metric)

Q = Design flow (gal/d; m3/d)

Partial-Mix Aerated Pond

The size of the partial-mix aerated pond site will vary with the climate; forexample, shorter detention times are used in warm climates For the purpose ofthis chapter, assume a 50-day detention time, a depth of 8 ft (2.5 m), and anorganic loading of 89 lb/ac·d (100 kg/ha·d) Expected effluent quality is BOD =

30 mg/L and TSS > 30 mg/L The site area can be calculated using Equation 2.5:

where

A pm = Aerated pond site area (ac; ha)

k = Factor (2.7 × 10–3, U.S units; 2.9 × 10–3, metric)

Q = Design flow (gal/d; m3/d)

2.1.2.2 Free Water Surface Constructed Wetlands

Constructed wetlands are typically designed to receive primary or secondaryeffluent, to produce an advanced secondary effluent, and to operate year-round

in moderately cold climates The detention time is assumed to be 7 days, the

16 Natural Wastewater Treatment Systems

depth is 1 ft (0.3 m), and the organic loading is <89 lb/ac·d (<100 kg/ha·d) The

expected effluent quality is BOD = 10 mg/L, TSS = 10 mg/L, total N < 10 mg/L

(during warm weather), and P > 5 mg/L The estimated site area given in Equation

2.6 does not include the area required for a preliminary treatment system before

the wetland:

where

A fws = Site area for free water surface constructed wetland (ac; ha)

k = factor (4.03 × 10–5, U.S units; 4.31 × 10–3, metric)

Q = Design flow (gal/d; m3/d)

2.1.2.3 Subsurface Flow Constructed Wetlands

Subsurface flow constructed wetlands generally require less site area for the same

flow than do free water surface wetlands The assumed detention time is 3 days,

the water depth is 1 ft (0.3 m), with a media depth of 1.5 ft (0.45 m); the organic

loading rate is <72 lb/ac·d (<80 kg/ha·d); and the expected effluent quality is

similar to the free water surface wetlands above:

where

A ssf = Site area for subsurface flow constructed wetland (ac; ha)

k = Factor (1.73 × 10–5, U.S units; 1.85 × 10–3, metric)

Q = Design flow (gal/d; m3/d)

2.1.2.4 Overland Flow Systems

The area required for an overland flow (OF) site depends on the length of the

operating season The recommended storage days for an overland flow system

for planning purposes can be estimated from Figure 2.1 The effective flow to

the OF site can then be estimated using Equation 2.8:

where

Q m = Average monthly design flow to the overland flow site (gal/mo; m3/mo)

q = Average monthly flow from pretreatment (gal/mo; m3/mo)

t s = Number of months storage is required

t a = Number of months in the operating season

The OF process can produce advanced secondary effluent from a primary effluent

or equivalent The expected effluent quality is BOD = 10 mg/L, TSS = 10 mg/L,

total N < 10 mg/L, and total P < 6 mg/L The site area given by Equation 2.9

includes an allowance for a 1-day aeration cell and for winter wastewater storage(if needed), as well as the actual treatment area, with an assumed hydraulicloading of 6 in./wk (15-cm/wk):

A of = (3.9 × 10–4)(Q m + 0.05qt s) (metric) (2.9a)

A of = (3.7 × 10–6)(Q m + 0.04qt s) (U.S.) (2.9b)where

A of = Overland flow project area (ac; ha)

Q m = Average monthly design flow to the overland flow site, gal/mo (m3/mo)

q = Average monthly flow from pretreatment, gal/mo (m3/mo)

t s = Number of months storage is required

2.1.2.5 Slow-Rate Systems

Slow-rate (SR) systems are typically nondischarging systems The size of theproject site will depend on the operating season, the application rate, and thecrop The number of months of possible wastewater application is presented inFigure 2.2 The design flow to the SR system can be calculated from Equation2.10 The land area will be based on either the hydraulic capacity of the soil orthe nitrogen loading rate The area estimate given in Equation 2.10 includes anallowance for preapplication treatment in an aerated pond as well as a winterstorage allowance The expected effluent (percolate) quality is BOD < 2 mg/L,TSS < 1 mg/L, total N < 10 mg/L (or lower if required), and total P < 0.1 mg/L:

A sr = (6.0 × 10–4)(Q m + 0.03qt s) (2.10a)

A sr = (5.5 × 10–6)(Q m + 0.04qt s) (2.10b)

FIGURE 2.1 Recommended storage days for overland flow systems.

2 to 5 days storage for operational flexibility

40

40

40 60

10

60

120

120 140

140 160

A sr = Slow rate land treatment project area (ac; ha)

Q m = Average monthly design flow to the SR site (gal/mo; m3/mo)

q = Average monthly flow from pretreatment (gal/mo; m3/mo)

t s = Number of months storage is required

2.1.2.6 Soil Aquifer Treatment Systems

Typically a soil aquifer treatment (SAT) or rapid infiltration system is a charging system Year-round operation is possible in all parts of the United States

nondis-so storage is not generally required The hydraulic loading rate, which depends

on the soil permeability and percolation capacity, controls the land area required.The expected percolate quality is BOD < 5 mg/L, TSS < 2 mg/L, total N > 10mg/L, and total P < 1 mg/L:

where

A sat = SAT project site area (ac; ha)

k = Factor (4.8 × 10–7 U.S units; 5.0 × 10–5, metric)

Q m = Average monthly design flow to the SAT site (gal/mo; m3/mo)

2.1.2.7 Land Area Comparison

The land area required for a community wastewater flow of 1 mgd (3785 m3/d)

is estimated using the above equations for each of the processes and is summarized

in Table 2.4 The three geographical locations in Table 2.4 reflect climate tions and the need for different amounts of storage: 5-month storage for SR and

varia-FIGURE 2.2 Approximate months per year that wastewater application is possible with

slow rate land treatment systems.

6 8

OF in the north, 3-month storage in the mid-Atlantic, and no storage in the warmclimate (south) No storage is expected for constructed wetlands, but the temper-ature of the wastewater is reflected in the larger land area requirements in thecolder north Allowances are included in the area requirements for unusable landand preliminary treatment.

be located on the maps Some options may be dropped from consideration because

no suitable sites are located within a reasonable proximity from the wastewatersource In the next step, local knowledge regarding land use commitments, costs,and the technical ranking procedure (described in the next section) are considered

TABLE 2.4

Treatment System

North [ac (ha)]

Mid-Atlantic [ac (ha)]

South [ac (ha)]

to determine which processes and sites are technically feasible A complex ing procedure is not usually required for pond and wetland systems, becauseclose proximity and access to the point of discharge are usually most important

screen-in site selection for these systems For land application systems for wastewaterand biosolids, the economics of conveyance to the potential site may competewith the physical and land use factors described in the next section

2.2.1 S ITE S CREENING P ROCEDURE

The screening procedure consists of assigning rating factors to each item for eachsite and then adding up the scores Those sites with moderate to high scores arecandidates for serious consideration, site investigation, and testing Among theconditions included in the general procedure are site grades, depth of soil, depth

to groundwater, and soil permeability (Table 2.6) Conditions for the wastewatertreatment concepts include land use (current and future), pumping distance, andelevation (Table 2.7) The relative importance of the various conditions in Table2.6 and Table 2.7 is reflected in the magnitude of the values assigned, so thelargest value indicates the most important characteristic The ranking for a specificsite is obtained by summing the values from Table 2.6 and Table 2.7 The highestranking site will be the most suitable The suitability ranking can be determinedaccording to the following ranges:

TABLE 2.5 Biosolids Loadings for Preliminary Site Area Determination

Optiona Application Schedule

Typical Loading Rate (Mg/ha)b

a See Chapter 9 for a detailed description of options.

b Metric tons per hectare (Mg/ha) × 0.4461 = lb/ac.

surface-applied biosolids Injection of liquid biosolids is acceptable on 6 to 12%slopes but is not recommended on higher grades without effective runoff control.The economical haul distance for a biosolids land application will depend onthe solids concentration and other local factors and must be determined on a case-by-case basis The values in Table 2.8 can be combined with the land use andland cost factors from Table 2.7 (if appropriate) to obtain an overall score for a

TABLE 2.6

Physical Rating Factors for Land Application of Wastewater

Condition Slow Rate Overland Flow

Soil Aquifer Treatment

b Soil depth to bedrock or impermeable barrier.

Source: Adapted from USEPA, Onsite Wastewater Treatment Systems Manual,

EPA/625/R-00/008, CERI, Cincinnati, OH, 2002.

potential biosolids application site These combinations produce the followingranges:

Agricultural Reclamation Type B

TABLE 2.7 Land Use and Economic Factors for Land Application

of Wastewater

Distance from wastewater source (km)

High density, residential or urban 0

Agricultural, or open space, for agricultural SR or OF 4 Forested:

Land cost and management

No land cost, farmer or forest company management 5 Land purchased, farmer or forest company management 3 Land purchased, operated by industry or city 1

Note: SR, slow rate; OF, overland flow.

The transport distance is a critical factor and must be included in the finalranking The rating values for distance given in Table 2.7 can also be used foragricultural biosolids operations In general, it is economical to transport liquidbiosolids (<7% solids) about 16 km (10 mi) from the source; for greater hauldistances, it is usually more cost effective to dewater and haul the dewateredbiosolids.

TABLE 2.8 Physical Rating Factors for Land Application of Biosolids

Condition Agriculturala Reclamation

c Soil depth to bedrock or impermeable barrier.

Source: Adapted from USEPA, Process Design Manual: Land Application of Sewage Sludge and Domestic Septage, EPA 625/R-95/001, CERI, Cincinnati,

OH, 1995.

TABLE 2.9 Rating Factors for Biosolids or Wastewater in Forests (Surface Factors)

Condition

Rating Valuea

Proceed-Madison, WI, September 1980.

Forested sites for either wastewater or biosolids are presented as a separatecategory in Table 2.9 and Table 2.10 In the earlier cases, the type of vegetation

to be used is a design decision to optimize treatment, and the appropriate tation is usually established during system construction It is far more commonfor forested sites to depend on preexisting vegetation on the site, so the type andstatus of that growth become important selection factors (McKim et al., 1982).The total rating combines values from Table 2.9 and Table 2.10 The final ranking,

vege-as with other methods, must include the transport distance; the values in Table2.7 can be used for wastewater systems

TABLE 2.10

Rating Factors for Biosolids or Wastewater in Forests (Subsurface Factors)

Condition

Rating Valuea Condition

Rating Valuea

Depth to seasonal groundwater (m) NRCS shrink–swell potential for the soil

a Total rating: 5–10, not suitable; 15–25, poor; 25–30, good; 30–45, excellent.

Source: Adapted from Taylor, G.L., in Proceedings of the Conference of Applied Research and Practice on Municipal and Industrial Waste, Madison, WI, September 1980.

2.2.2 C LIMATE

The regional climate has a direct effect on the potential biosolids managementoptions, as shown in Table 2.11 Climatic factors are not included in the ratingprocedure for wastewater systems, because seasonal constraints on operations arealready included as a factor in the land area determinations Seasonal constraintsand the local climate are important factors in determining the design hydraulicloading rates and cycles for wastewater systems, as well as the length of theoperating season and stormwater runoff conditions for all concepts The pertinentclimatic data required for the design of both wastewater and biosolids systemsare listed in Table 2.12 At least a 10-year return period is recommended, althoughsome agencies require a 100-year return period (see NOAA references)

2.2.3 F LOOD H AZARD

The location of wastewater and biosolids systems within a flood plain can beeither an asset or a liability, depending on the approach used for planning anddesign Flood-prone areas may be undesirable because of variable drainage char-acteristics and potential flood damage to the structural components of the system

On the other hand, flood plains and similar terrain may be the only deep soils inthe area If permitted by the regulatory authorities, utilization of such sites forwastewater or biosolids can be an integral part of a flood-plain management plan.Off-site storage of effluent or biosolids can be a design feature to allow the site

to flood as needed

Maps of flood-prone areas have been produced by the U.S Geological Survey(USGS) in many areas of the United States as part of the Uniform NationalProgram for Managing Flood Losses The maps are based on the standard 7.5-minute USGS topographic sheets and identify areas with a potential of a 1-in-100

TABLE 2.11

Climatic Influences on Land Application of Biosolids

Source: Adapted from USEPA, Process Design Manual: Land Application of Municipal Sludge,

EPA 625/1-83-016, CERI, Cincinnati, OH, 1983