Natural Wastewater Treatment Systems - Chapter 2 potx

11 2 Planning, Feasibility Assessment, and Site Selection When conducting a wastewater treatment and reuse/disposal planning study, it is important to evaluate as many alternatives as possible to ensure that the most cost-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 nor economical, however, to conduct extensive field investigations for every process, at every potential site, during planning. This chapter provides a sequential approach that first determines potential feasibility and the necessary land require- ments and site conditions of each alternative. The second step evaluates each site coupled with a natural treatment process based on technical and economic factors and selects one or more for detailed investigation. The final step involves detailed field 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 discharging and nondischarging systems. Discharging systems would include those with a surface water discharge, such as treatment ponds, constructed wetlands, and overland flow land treatment. Underdrained slow rate or soil aquifer treatment (SAT) systems may also have a surface water discharge that would be permitted under 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 for nondischarging systems. Design features and performance expectations for both types of systems are presented in Table 2.1, Table 2.2, and Table 2.3. Special site requirements are summarized in Table 2.1 and Table 2.2 for each type of system for planning purposes. It is presumed that the percolate from a nondischarging system mingles with any groundwater that may be present. The typical regulatory DK804X_C002.fm Page 11 Friday, July 1, 2005 3:25 PM © 2006 by Taylor & Francis Group, LLC 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 discharge from underdrains, recovery wells, or cutoff ditches. For example, the large SR system at Muskegon County, Michigan, has underdrains with a surface water discharge. For the forested SR system at Clayton County, Georgia, the subflow from the wastewater application leaves the site and enters the local streams. Although the subflow does emerge in surface streams, which are part of the community’s drinking water supplies, the land treatment system is not considered to be a discharging system as defined by the U.S. Environmental Protection Agency (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 potential sites are based on the analysis of maps and other information. The requirements shown in Table 2.1 and Table 2.2, along with an estimate of the land area required for each of the methods, are considered during this procedure. The sources of information and type of information needed are summarized in Table 2.3. TABLE 2.1 Special Site Requirements for Discharge Systems Concept Requirement 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 DK804X_C002.fm Page 12 Friday, July 1, 2005 3:25 PM © 2006 by Taylor & Francis Group, LLC Planning, Feasibility Assessment, and Site Selection 13 TABLE 2.2 Special Site Requirements for Nondischarging System Concept Requirement 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 Information Source Information Items Topographic maps Elevations, slope, water and drainage features, building and road locations Natural Resources Conservation Service soil surveys Soil type, depth and permeability, depth to bedrock, slope Federal Emergency Management Agency (FEMA) maps Flood hazard Community maps Land use, water supply, sewerage systems National Oceanic and Atmospheric Administration (NOAA) Climatic data U.S. Geological Survey (USGS) reports and maps Geologic data, water quality data DK804X_C002.fm Page 13 Friday, July 1, 2005 3:25 PM © 2006 by Taylor & Francis Group, LLC 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 in Table 2.1 and Table 2.2 to determine, with a study of the maps, whether suitable sites exist for the process under consideration. These preliminary area estimates are very conservative and are intended only for this preliminary evaluation. These estimates 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 conventional approaches. The area estimate for pond systems will depend on the effluent quality required (as defined by biochemical oxygen demand [BOD] and total suspended solids [TSS]), on the type of pond system proposed, and on the climate in the particular geographic location. A facultative pond in the southern United States will require less area than the same process in Canada. The equations given below are for total project area and include an allowance for roads, levees, and unusable portions of the site. Oxidation Ponds The area for an aerobic pond assumes a depth of 3 ft (1 m), a warm climate, a 30-day detention time, an organic loading rate of 80 lb/ac·d (90 kg/ha·d), and an effluent quality of 30 mg/L BOD and >30 mg/L TSS. The planning area required is calculated using Equation 2.1: A pm = ( k )( Q ) (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; m 3 /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 effluent BOD of 30 mg/L, and TSS > 30 mg/L. The area required is: A fc = ( k )( Q ) (2.2) 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; m 3 /d). DK804X_C002.fm Page 14 Friday, July 1, 2005 3:25 PM © 2006 by Taylor & Francis Group, LLC 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 organic loading of 50 lb/ac·d (56 kg/ha·d); the expected effluent quality is BOD = 30 mg/L and TSS > 30 mg/L. The area required is: A fw = ( k )( Q ) (2.3) 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; m 3 /d). Controlled-Discharge Ponds Controlled-discharge ponds are used in northern climates to avoid winter dis- charges and in warm climates to match effluent quality to acceptable stream flow conditions. The typical depth is 5 ft (1.5 m), maximum detention time is 180 days, and the expected effluent quality is BOD < 30 mg/L and TSS < 30 mg/L. The required site area is: A cd = ( k )( Q ) (2.4) 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; m 3 /d). Partial-Mix Aerated Pond The size of the partial-mix aerated pond site will vary with the climate; for example, shorter detention times are used in warm climates. For the purpose of this chapter, assume a 50-day detention time, a depth of 8 ft (2.5 m), and an organic 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: A pm = ( k )( Q ) (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; m 3 /d). 2.1.2.2 Free Water Surface Constructed Wetlands Constructed wetlands are typically designed to receive primary or secondary effluent, 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 DK804X_C002.fm Page 15 Friday, July 1, 2005 3:25 PM © 2006 by Taylor & Francis Group, LLC 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: A fws = ( k )( Q ) (2.6) 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; m 3 /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: A ssf = ( k )( Q ) (2.7) 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; m 3 /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: Q m = q + (t s )(q)/t a (2.8) where Q m =Average monthly design flow to the overland flow site (gal/mo; m 3 /mo). q =Average monthly flow from pretreatment (gal/mo; m 3 /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 DK804X_C002.fm Page 16 Friday, July 1, 2005 3:25 PM © 2006 by Taylor & Francis Group, LLC Planning, Feasibility Assessment, and Site Selection 17 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 hydraulic loading 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 (m 3 /mo). q =Average monthly flow from pretreatment, gal/mo (m 3 /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 the project site will depend on the operating season, the application rate, and the crop. The number of months of possible wastewater application is presented in Figure 2.2. The design flow to the SR system can be calculated from Equation 2.10. The land area will be based on either the hydraulic capacity of the soil or the nitrogen loading rate. The area estimate given in Equation 2.10 includes an allowance for preapplication treatment in an aerated pond as well as a winter storage 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 60 120 120 120 120 140 140 140 160 160 160 160 160 100 100 100 60 60 60 80 80 DK804X_C002.fm Page 17 Friday, July 1, 2005 3:25 PM © 2006 by Taylor & Francis Group, LLC 18 Natural Wastewater Treatment Systems where A sr = Slow rate land treatment project area (ac; ha). Q m =Average monthly design flow to the SR site (gal/mo; m 3 /mo). q =Average monthly flow from pretreatment (gal/mo; m 3 /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 nondis- charging system. Year-round operation is possible in all parts of the United States 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 > 10 mg/L, and total P < 1 mg/L: A sat = (k)(Q) (2.11) 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; m 3 /mo). 2.1.2.7 Land Area Comparison The land area required for a community wastewater flow of 1 mgd (3785 m 3 /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 varia- tions and the need for different amounts of storage: 5-month storage for SR and FIGURE 2.2 Approximate months per year that wastewater application is possible with slow rate land treatment systems. 6 8 10 8 6 12 12 10 8 6 DK804X_C002.fm Page 18 Friday, July 1, 2005 3:25 PM © 2006 by Taylor & Francis Group, LLC Planning, Feasibility Assessment, and Site Selection 19 OF in the north, 3-month storage in the mid-Atlantic, and no storage in the warm climate (south). No storage is expected for constructed wetlands, but the temper- ature of the wastewater is reflected in the larger land area requirements in the colder north. Allowances are included in the area requirements for unusable land and preliminary treatment. 2.1.2.8 Biosolids Systems The land area required for biosolids land application systems is summarized in Table 2.5. The actual rates depend on the climate and the biosolids characteristics, as discussed in Chapter 9. 2.2 SITE IDENTIFICATION The information presented or developed in the previous sections is combined with maps of the community area to determine if feasible sites for wastewater treatment or biosolids land application exist within a reasonable distance. It is possible that a community or industry may not have suitable sites for all the natural system options listed in Table 2.1 and Table 2.2. All suitable sites should be located on the maps. Some options may be dropped from consideration because no suitable sites are located within a reasonable proximity from the wastewater source. 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 Planning Level Land Area Estimates for 1-mgd (3785-m 3 /d) Systems Treatment System North [ac (ha)] Mid-Atlantic [ac (ha)] South [ac (ha)] Pond systems: Oxidation NA NA 30 (12.1) Facultative 157 (63.6) 102 (41.3) 48 (19.3) Controlled discharge 152 (61.7) 152 (61.7) 152 (61.7) Partial-mix 48 (19.3) 36 (14.5) 27 (11.0) Free water surface constructed wetlands 56 (22.7) 47 (19.1) 40 (16.3) Subsurface flow constructed wetlands 24 (9.7) 20 (8.2) 17 (7.0) Slow rate 311 (126) 240 (97) 168 (68) Overland flow 215 (87) 160 (65) 111 (45) Soil aquifer treatment 14 (5.7) 14 (5.7) 14 (5.7) Note: 1 ac = 0.404 ha. DK804X_C002.fm Page 19 Friday, July 1, 2005 3:25 PM © 2006 by Taylor & Francis Group, LLC 20 Natural Wastewater Treatment Systems to determine which processes and sites are technically feasible. A complex screen- ing procedure is not usually required for pond and wetland systems, because close proximity and access to the point of discharge are usually most important in site selection for these systems. For land application systems for wastewater and biosolids, the economics of conveyance to the potential site may compete with the physical and land use factors described in the next section. 2.2.1 SITE SCREENING PROCEDURE The screening procedure consists of assigning rating factors to each item for each site and then adding up the scores. Those sites with moderate to high scores are candidates for serious consideration, site investigation, and testing. Among the conditions included in the general procedure are site grades, depth of soil, depth to groundwater, and soil permeability (Table 2.6). Conditions for the wastewater treatment concepts include land use (current and future), pumping distance, and elevation (Table 2.7). The relative importance of the various conditions in Table 2.6 and Table 2.7 is reflected in the magnitude of the values assigned, so the largest value indicates the most important characteristic. The ranking for a specific site is obtained by summing the values from Table 2.6 and Table 2.7. The highest ranking site will be the most suitable. The suitability ranking can be determined according to the following ranges: Low suitability <18 Moderate suitability 18–34 High suitability 34–50 For land application of biosolids, a similar matrix of factors can be arrayed for each potential site using the rating factors shown in Table 2.8. The rating factors for forested sites are presented in Table 2.9 and Table 2.10. The restrictions on liquid biosolids (<7%) in Table 2.8 are intended to control runoff or erosion of TABLE 2.5 Biosolids Loadings for Preliminary Site Area Determination Option a Application Schedule Typical Loading Rate (Mg/ha) b Agricultural Annual 10 Forest 5-yr intervals 45 Reclamation One time 100 Type B Annual 340 a See Chapter 9 for a detailed description of options. b Metric tons per hectare (Mg/ha) × 0.4461 = lb/ac. DK804X_C002.fm Page 20 Friday, July 1, 2005 3:25 PM © 2006 by Taylor & Francis Group, LLC [...]... caseby-case basis The values in Table 2. 8 can be combined with the land use and land cost factors from Table 2. 7 (if appropriate) to obtain an overall score for a © 20 06 by Taylor & Francis Group, LLC DK804X_C0 02. fm Page 22 Friday, July 1, 20 05 3 :25 PM 22 Natural Wastewater Treatment Systems TABLE 2. 7 Land Use and Economic Factors for Land Application of Wastewater Condition Rating Value Distance from wastewater. .. Agricultural 30 3 20 –30 3 50 1 30–50 2 40 1 25 –40 2 35 0 0–1 2 2–6 4 7–35 6 Distance to surface waters (m) 15–30 1 30–60 2 >60 3 Adjacent land use High-density residential 1 Low-density... interbedding of fine- and coarse-grained layers tends to restrict vertical flow Typical values are given in Table 2. 19 © 20 06 by Taylor & Francis Group, LLC DK804X_C0 02. fm Page 35 Friday, July 1, 20 05 3 :25 PM Planning, Feasibility Assessment, and Site Selection 35 TABLE 2. 19 Horizontal Permeability Kh (m/d) Kh /Kv Comments 42 2.0 Silty soil 75 2. 0 — 56 4.4 — 100 7.0 Gravelly 72 20.0 Near terminal morain 72 10.0... distribution in the natural soil can be changed easily by the use of soil amendments such as lime or gypsum © 20 06 by Taylor & Francis Group, LLC DK804X_C0 02. fm Page 32 Friday, July 1, 20 05 3 :25 PM 32 Natural Wastewater Treatment Systems TABLE 2. 16 Interpretation of Soil Chemical Tests Parameter and Test Result Interpretation pH of saturated soil paste 35 Forest only, 0 NS NS 0.3–0.6 . Onsite Wastewater Treatment Systems Manual, EPA/ 625 /R-00/008, CERI, Cincinnati, OH, 20 02. DK804X_C0 02. fm Page 21 Friday, July 1, 20 05 3 :25 PM © 20 06 by Taylor & Francis Group, LLC 22 Natural Wastewater. EPA 625 /R-95/001, CERI, Cincinnati, OH, 1995. DK804X_C0 02. fm Page 23 Friday, July 1, 20 05 3 :25 PM © 20 06 by Taylor & Francis Group, LLC 24 Natural Wastewater Treatment Systems TABLE 2. 9 Rating. Madison, WI, September 1980. DK804X_C0 02. fm Page 25 Friday, July 1, 20 05 3 :25 PM © 20 06 by Taylor & Francis Group, LLC 26 Natural Wastewater Treatment Systems 2. 2 .2 CLIMATE The regional climate has