Land Treatment Systems

Land application of wastewater is perhaps the oldest method for disposal and treatment of wastewaters. Early systems were used in England as

"Land Farms," which received untreated wastewater and night soil from nearby communities. Today, land application systems have included application to edible and nonedible crops, to rangelands, to forests and wood plantations, to recreational areas including parks and golf courses, and to disturbed lands such as mine spoil sites.

Land application of wastewater also plays a role in recharging ground-water and recycling fresh water. On the average, water that is used once and then discharged to the ocean would not return as rain on land for about 2,600 years. This fact has shed a spotlight on the wastewater reuse issue. Wastewater reuse in agriculture and other fields is not new; however, increasing environmental awareness has made the reuse of wastewater, even after careful treatment, a tainted prospect. This level of concern about reuse of wastewater is not unreasonable, given the checkered history of wastewater disposal throughout human development across the world. But as wastewater treatment technologies advance and quality of treated effluents steadily improves, the land application of treated wastewater from food and agriculture becomes a cost-effective alternative to discharging into the surface water, including oceans.

There are two major categories of reuse of wastewater, which have been practiced throughout the world: potable use and nonpotable use. The potable use of wastewater mainly includes injecting reclaimed water to the drinking water supply after multiple levels of treatments, or using natural systems (including land applications) to treat wastewater directly. Nonpotable uses of wastewater are many: direct irrigation of agriculture fields using food wastewater with low BOD5 and TSS; irrigation of parks, forests, or golf courses with low-load wastewater; and use for aquaculture are the most promising examples. In many areas of the world, wastewater reuse has been practiced using a combination of treatment technologies that achieve a very high degree of treatment. Many states in the western U.S. have, over the past 20 years, been treating wastewater to tertiary treatment standards and then allowing the wastewater to be reused for irrigation or for recharge to groundwater aquifers. Although this is an effective method for many arid regions in the western U.S., it is very expensive and is rarely practiced in other regions of the world.

The land application of wastewaters is not without risk of contamination of soil and groundwater beneath it. The challenge is to utilize the physico-chemical and biological properties of the soil as an acceptor for the waste-water streams without undue effect on the crops that are to be grown or to the ecosystem of which the land is a part, to the characteristics of the soil,

Figure 6.6. A schematic illustration of land applications of wastewaters.

Deep seepage

Figure 6.6. A schematic illustration of land applications of wastewaters.

and to the quality of the groundwater and the surface water. The wastewater and the soil to which the wastewater is applied should be managed as an integrated system to obtain the best outcome of the application.

Land application of wastewaters incorporates organic and inorganic materials into the soil for recycle and reuse. The assimilative capacity of a soil is dependent on its characteristics and environmental conditions. The maximum capacity of a soil represents the maximum wastewater loading of the soil. This is true for raw wastewaters as well as treated wastewaters. Each application site will have a controlling parameter dependent on characteristics of the wastewater applied and characteristics of the soil, and most importantly, the environmental ramification. In most cases, permits are required for applying wastewater to the land.

Fig. 6.6 is a schematic diagram of the use of land for plantation and wastewater land applications. As shown in the diagram, many factors are involved in the overall effect of the water cycle on plants, including land application of wastewater. In most cases, the treated wastewater is applied to the land surface via furrow-flood, sprayer, or drip irrigation. BOD5, TSS, and fecal coliform (FC) are partially removed in the conventional

Table 6.5. Characteristics of land wastewater treatment systems (adapted from USEPA, 1981, 1984).

Feature

Slow Rate

Overland Flow

Rapid Infiltration

Application

Sprinkler or

Sprinkler or

Usually Surface

Techniques

Surfacea

Surface

Annual loading

0.5-6

3-20

6-125

rate, m

Field area re

23-280

6.5-44

3-23

quired, hab

Typical weekly

1.3-10

6-40c

10-240

loading rate, cm

Minimum preap-

Primary sedimen-

Grit removal and

Primary sedimen-

plication pro-

tationd

screening

tatione

vided in the

United States

Disposition of

Evapotranspira-

Mainly surface

Percolation

applied waste-

tion and perco-

runoff and évapo

water

lation

transpiration

Need for

Required

Required

Optional

vegetation

aIncludes ridge-and-furrow and border strip.

bField area in hectares not including buffer area, roads, or ditches for 3,785 m3/d (l Mgal/d) flow.

cRange includes raw wastewater to secondary effluent, higher rates for higher level of preapplication treatment.

dWith restricted public access; crops not for direct human consumption. eWith restricted public access.

wastewater treatment steps; the land application system removes additional BOD5, TSS, and FC as well as nitrogen and phosphorus.

Various designs of land application systems have been developed, including application of wastes to the soil-surface using slow rate, overland flow, and rapid infiltration land treatment systems, and to the subsurface using leaching fields and absorption beds. The suitability of a particular system depends on site characteristics, including soil properties, ground topography such as slope and relief, local hydrology, groundwater depth and quality, land use, climatic factors such as temperature, precipitation, evapotranspiration, wind, length of growing season, and expected waste loading rates, as well as consideration of possible social and economic

Table 6.6. Comparison of site characteristics for land treatment processes (adapted from USEPA, 1981, 1984).

Characteristics Slow Rate Overland Flow Rapid Infiltration

Slope

Soil permeability

Depth to ground water

Climatic restrictions

Less than 20% on cultivated land; less than 40% on noncul-tivated land Moderately slow and moderately rapid

Storage often needed for cold weather and during heavy preci-

Finish slopes

Slow (for cold clays, silts, soils with impermeable barriers) Not critical0

Storage needed for cold weather

Not critical; excessive grades require much earthwork

Rapid (sands, sandy loams)

1 m during flood cycleb; 1.5-3 m during drying cycle None (possibly modify storage usually needed in cold weather)

pitation aSteeper grades might be feasible at reduced hydraulic loadings. bUnder-drains can be used to maintain this level at sites with high ground water table.

cImpact on ground water should be considered for more permeable soils.

constraints. Tables 6.5 and 6.6 list the characteristics of three major types of land application systems and comparison of site characteristics for natural systems.

In designing any land application system, several common attributes of land application systems are often included in overall considerations:

• Public health

• Groundwater issues

• Site evaluation and selection

• Crop selection and management if needed

• Preapplication treatment

• Distribution methods

• Design hydraulic loading rate

• Application rate

• Climatic consideration and storage

• BOD5 loading rate if needed

• Nitrogen removal

• Limiting nitrogen loading rate if required

• Suspended solid removal

• Phosphorus removal

• Land requirements

• Recovery of effluent from land application if possible Slow-rate (SR) systems

Slow rate systems are the prevailing form of land application system in use today and involve the application of pretreated wastewaters to vegetated soil to provide treatment of the wastewater and meet the growth needs of the existing vegetation through evapotranspiration and percolation (see Fig. 6.7). The systems are similar to those found in common agricultural irrigation. The annual loading rate of the systems ranges from 0.5 to 6 m/year. There are roughly three types of slow-rate systems categorized by the objectives of applications of the slow-rate systems (Parany-chianakis et al., 2006):

• Type I is based on the objective of reuse of wastewater for crop and vegetation growth. The application of pretreated wastewater can be achieved through the uses of sprinklers, ridge-and-furrow, border strip flooding, and other surface or subsurface methods.

• Type II focuses on treatment of wastewater not on reuse of wastewater. When the primary objective of the SR process is treatment, the hydraulic loading is usually limited, either by the hydraulic capacity of the soil or the nitrogen removal capacity of the soil-vegetation matrix. Under-drains are sometimes needed for development of sites with high groundwater tables, or where perched water tables or impermeable layers prevent deep percolation. Perennial grasses are often chosen for the vegetation because of their high nitrogen uptake, a longer wastewater application season, and the avoidance of annual planting and cultivation. Corn and other crops with higher market values are also grown on systems where treatment is the major objective.

PERC9LATION

(a) APPLICATION PATHWAY

Figure 6.7. (a)(b)(c) A schematic diagram of a slow-rate wastewater treatment system (USEPA, 1981).

Figure 6.7. (a)(b)(c) A schematic diagram of a slow-rate wastewater treatment system (USEPA, 1981).

• Filter beds treat wastewaters through the actions of percolation and filtration of the beds. The treated effluents are collected by a vast underground drain system. Filter bed systems are effective in nutrient removal and suitable for population-intensive areas such as urban areas. Specifically modified filter beds can be used for treating various indus trial wastewaters such as those from animal farming and dairy operations (Jayawardane et al., 1997).

SR system design

Slow-rate systems must be combined with other processes in order to produce a complete wastewater treatment. Preapplication treatment is often required to ensure protection of public health, nuisance control, and distribution systems constraints. The main concern is the pathogen content in wastewaters. There are several common processes that can be used: fine screening, primary sedimentation, and pH adjustment, as well as biological processes such as stabilization ponds.

Crop selection is also important for slow-rate land systems for application of pretreated wastewaters, particularly for water reuse. The choice of individual crops depends on the nitrogen content and revenue for Type I systems and maximum nutrient uptake for Type II. Revenue-generating field crops such as corn, soybean, sugar beets, barley, and wheat are all good choices. For forage crops, alfalfa, quack grass, ryegrass, orchard grass, bromegrass, and Kentucky bluegrass are suitable crops for maximum removal of nutrients.

For hydraulic loading for Type I systems, the water balance equation is used to determine the hydraulic rate (Equation 6.4):

Lw = wastewater hydraulic loading rate based on soil permeability ET = evapotranspiration rate, in/mo Pr = precipitation rate, in/mo Pw = percolation, in/mo

The ET is normally the monthly average ET rate of the selected crop and is determined from the historical evaporation data (at least 15 consecutive years). The value of Pr should be determined from a frequency analysis of wetter than normal years (using 10 years as a basis). The design percolation rate Pw is estimated to be 4-10% of the measured field test data or published data based on wet:dry ratio, thus is determined by the following (Equation 6.5):

where:

Pw = design percolation rate, cm/d or in/d K = permeability of limiting soil layer, cm/h or in/h Adjustment Factor = 4% to 10% to account for wet:dry ratio and ensure a conservative and safe value for infiltration of wastewaters

For hydraulic loading based on nitrogen limits, the nitrogen balance for the SR system is the following (Equation 6.6):

where:

Ln = mass loading of nitrogen, kg/ha-yr or lb/ac-yr U = crop uptake, kg/ha-yr or lb/ac-yr f = fraction of applied nitrogen lost to denitrificaton, volatilization, and soil storage

A = unit conversion factor, 0.1 for metric units and 2.7 in imperial units Cp = percolation nitrogen concentration, mg/l, usually set at 10 mg/l due to the limiting nitrogen concentration Pw = percolation rate, cm/yr or ft/yr

Crop uptake can be found from the literature and the f value is based on wastewater characteristics and climate: 0.5 to 0.8 if the BOD5:nitrogen ratio is 5 or more, 0.25 to 0.5 for primary treatment effluents in municipal wastewater plants, and 0.15 to 0.25 should be used for effluents from secondary treatment processes of municipal wastewater treatment plants; a value of 0.1 is used for effluents from advanced wastewater treatment processes.

For Type II slow-rate systems, hydraulic loading rate is based on water balance and is expressed as the following (Equation 6.7):

Lw = hydraulic loading rate

IR = crop irrigation requirement

Pr = precipitation, cm/yr, m/yr or in/yr

The crop irrigation requirement depends on the crop ET, the irrigation efficiency, and the leaching requirement. Incorporating these three factors into Equation 6.7 yields the following (Equation 6.8):

where:

LR = leaching requirement Pr = precipitation, cm/yr, m/yr or in/yr ET = evapotranspiration rate, inch/month E = efficiency of the irrigation systems

The leaching factor ranges from 0.05 to 0.30 depending on the crop, the amount of precipitation, and the total dissolved solids in the wastewater. For the total dissolved solids of 400 mg/l or more, LR is in the range from 0.1 to 0.2. The efficiency of the irrigation system is 0.65 to 0.75 for surface irrigation systems, 0.7 to 0.8 for sprinklers, and 0.9 to 0.95 for drip irrigation systems.

Overland-flow systems

Overland-flow is essentially a biological treatment process in which wastewater is treated as it flows over the upper reaches of sloped terraces and is allowed to flow across the vegetated surface to runoff collection ditches. Unlike slow-rate systems, overland-flow systems are designed to facilitate the runoff of wastewaters. In order to ensure a runoff, the soil on the slope should be either impervious to water or slowly permeable to limit percolation.

A schematic view of the overland-flow treatment is shown in Fig. 6.8 (a), and a pictorial view of a typical system is shown in Fig. 6.8(b). As shown in Fig. 6.8(a), there is relatively little percolation involved either because of an impermeable soil or a subsurface barrier to percolation. Wastewaters are either sprinkler-applied, fan-sprayed, or surface-applied (e.g., gated pipe) to the top of the slope. The treatment of the wastewater occurs as the flow runs down the graded land. The slopes are normally 2-8% grade and 30-61 m (100-200 ft) in length, as tabulated in Tables 6.5 and 6.6.

The objectives of overland-flow systems are to achieve high effluent quality by applying to the land pretreated wastewaters and to remove ni-

(ASTEIATiR

IRASS AND

IRASS AND

(ASTEIATiR

PERCILATION (a) HYDRAULIC PATHVA T

(t) MtTllllU IIII if JPiimEfi IPrLlOITIIjl

Figure 6.8. (a)(b) A schematic diagram of an overland flow wastewater treatment system (USEPA, 1984).

(t) MtTllllU IIII if JPiimEfi IPrLlOITIIjl

Figure 6.8. (a)(b) A schematic diagram of an overland flow wastewater treatment system (USEPA, 1984).

trogen, phosphorus, BOD5, and suspended solids. The pretreatment includes grit and fine screening, primary sedimentation, secondary processes, or pond systems. It is also feasible for treating high-strength food processing wastewaters. The collected overland-flow treated water from the ditches is discharged to surface waters.

The primary removal mechanisms for organics and suspended solids are biochemical oxidation, sedimentation, adsorption, and filtration. Nitrogen removal is a combination of plant uptake, denitrification, and volatilization of ammonia nitrogen. The dominant mechanism in a particular site will depend on the forms of nitrogen present in the wastewater, the amount of carbon available, the temperature, and the loading rates and schedules of wastewater application. Permanent nitrogen removal by the plants is possible only if the crop is harvested and removed from the field. Ammonia volatilization can be significant if the pH of the wastewater is above 7. Nitrogen removals usually range from 75-90%, with the form of runoff nitrogen dependent on temperature and on application loading rates and schedule. Because microbial activities for denitrification occur at or near the soil surface, the denitrification reactions are adversely affected by cold weather; the same problem also afflicts plant uptake of ammonia as the majority of crops die off or are in the dormant state in cold winter. Less removal of nitrate and ammonium may result from cold temperature in winter in many nontropical or non-subtropical regions of the world.

Phosphorus is removed by adsorption and precipitation. Treatment efficiencies are somewhat limited because of the limited contact between the wastewater and the adsorption sites within the soil. Phosphorus removals usually range from 50-70% on a mass basis. Increased removals may be obtained by adding coagulants such as alum or ferric chloride to the wastewater just prior to application on the slope.

Overland-flow system design

The soil permeability is an important parameter for designing an overland-flow system, because runoff of wastewater along the slope of the land is required. The best sites for overland-flow systems have soil permeabilities less than 0.5 cm/h (0.2 in/h) or less. The high permeability soils can be compacted mechanically to reduce permeability to acceptable levels. Low temperature and rainfall can affect overland-flow systems and as such, the wastewater application may be curtailed or ceased; the wastewater is stored.

The hydraulic loading rate of wastewater is empirically selected to be from 2 to 10 cm/d (0.8 to 4 in/d). Because there is little percolation, the application rate (see the definition below) is more relevant a performance parameter than hydraulic load rate. The two rates are related in the following expression (Equation 6.9) (Reed et al., 1995):

Lw = hydraulic loading rate, cm/d or in/d q = application rate per unit width of the slope, m3/h-m or gal/min-ft p = application period, h/d (ranging from 6 to 12 hours per day; 8 h/d is selected for the purpose of convenience) z = distance down-slope, m or ft (ranging from 30 to 60 m or 100 to 200 ft; for surface application, the length should be 30 to 45 m or 100 to 150 ft)

The application rate is also related to slope length and BOD removal through the following first-order reaction equation (Equation 6.10) (Reed et al., 1995):

Cz = BOD5 concentration of surface flow at a distance (z) down-slope, mg/l C = background BOD5 at the end of the slope C0 = BOD5 concentration of applied wastewater, mg/l z = distance down-slope, m or ft

A = empirically determined coefficient dependent on the value of q q = application rate, m3/h-m or gal/min-ft n = empirically determined exponents (<1) k = empirically determined rate constant

For overland-flow treatment of high-strength industrial wastewater such as some food processing wastewaters, the BOD5 loading must be considered. In order to avoid an excessive amount of organic loading on the slope, which leads to anaerobic activities due to exhausting of oxygen at or near the soil surface, the BOD5 loading rate is controlled at the following (Equation 6.11):

Lbod = BOD5 loading rate, kg/ha-d or lb/ac-d

B = conversion factor = 0.1 for metric units or 0.225 for imperial units

Lw = hydraulic loading rate, cm/d or in/d

= qPWm/z q = application rate, m3/h-m or gal/min-ft P = application period, h/d W = width of application slope, m or ft z = distance down-slope, m or ft m = conversion factor = 100 cm/m or 96.3 for imperial units C0 = BOD5 concentration of applied wastewater, mg/l

When BOD5 concentration of wastewater exceeds 800 mg/l, the waste-water has been diluted in order to avoid excessive anaerobic activities.

Land requirements for overland-flow systems depend on the flow, the application rate, and the application period. The required surface area for overland-flow treatment is the following (Equation 6.12):

where:

As = field surface area required, ha or ac Q = wastewater flow rate, m3/d or gal/min z = distance down-slope or slope length, m or ft q = application rate, m3/h-m or gal/min-ft P = application period, h/d

C = conversion factor = 10,000 m2/ha or 726 for imperial units

If wastewater storage is anticipated, the field area becomes the following (Equation 6.13):

where:

As = field surface area required, ha or ac

Vs = net loss or gain in storage volume due to precipitation, evaporation, and seepage, m3/yr or ft3/yr D = number of operating days per year Lw = design hydraulic loading, cm/d or in/d

C' = conversion factor = 100 for metric units and 3630 for imperial units

If BOD5 loading is a limiting factor, the field area is expressed as the following (Equation 6.14):

As = field surface area required, ha or ac C0 = BOD5 concentration of applied wastewater, mg/l C" = conversion factor = 0.1 for metric units and 6.24 X 10~5 for imperial units

Llbod = limiting BOD5 loading rate = 100 kg/ha-d or 89 lb/ac-d Qa = design flow rate to the overland-flow site, m3/d or ft3/d

Rapid-infiltration systems

The objective of the application of rapid-infiltration systems is to recharge or store renovated water in the underground aquifer and, in some cases, recharge surface waters using under-drains or wells to channel the water to the adjacent surface water body. In rapid-infiltration land treatment, most of the applied wastewater percolates through the soil, and the treated effluent drains naturally to surface waters or joins the ground water. The wastewater is applied to moderately and highly permeable soils (such as sands and loamy sands), by spreading in basins or by sprinkling, and it is treated as it travels through the soil matrix. Vegetation is not usually planned, but there are some exceptions, and emergence of weeds and grasses usually does not cause problems.

The schematic view in Fig. 6.9(a) shows the typical hydraulic pathway for rapid infiltration systems. A much greater portion of the applied wastewater percolates to the groundwater than with SR land treatment. There is little or no consumptive use by plants. Evaporation ranges from about 0.6 in/yr for moderately permeable soil (2 ft/yr for highly permeable soil) for cool regions to 2 in/yr for moderately permeable soil (6 ft/yr for highly permeable soil) for hot arid regions. This is usually a small percentage of the hydraulic loading rates.

In many cases, recovery of renovated water is an integral part of the system. This can be accomplished using under-drains or wells, as shown in Fig. 6.9(b). In some cases, the water drains naturally to an adjacent surface water body, as in Fig. 6.9(c). Such systems can provide a higher level iff* ice ntmmi iff* ice ntmmi

(i) mrmiiL rc Ptihiu

ILttHtl 111)41 *CC3TI HF1 lirrt

ILttHtl 111)41 *CC3TI HF1 lirrt

VMCUtmt (ILLS

{9) IL COTE H r ilTHiilS

VMCUtmt (ILLS

{9) IL COTE H r ilTHiilS

FLOOIING ItSIN

FLOOIING ItSIN

(c) NATURAL DRAI NICE INTO SURFACE (ITERS

Figure 6.9. (a)(b)(c) A schematic diagram of a rapid infiltration wastewater treatment system (USEPA, 1984).

(c) NATURAL DRAI NICE INTO SURFACE (ITERS

Figure 6.9. (a)(b)(c) A schematic diagram of a rapid infiltration wastewater treatment system (USEPA, 1984).

of treatment than most mechanical-intensive systems for discharging to the same surface water.

Removals of wastewater constituents by the filtering and straining action of the soil are superb. Suspended solids, BOD5, and fecal coliforms are almost completely removed. Nitrification of the applied wastewater is essentially complete when appropriate hydraulic loading cycles are used. Hence, for communities that have ammonia standards in their discharge requirements, rapid-infiltration can provide an effective way to meet such standards. Generally, nitrogen removal averages 50% unless specific operating procedures are established to maximize denitrification. These procedures include optimizing the application cycle, recycling the portions of the renovated water that contain high nitrate concentrations, reducing the infiltration rate, and supplying an additional carbon source. Using these procedures in soil column studies, average nitrogen removals of 80% have been achieved.

Phosphorus removals can range from 70-99%, depending on the physical and chemical characteristics of the soil. As with slow-rate systems, the primary removal mechanism is adsorption with some chemical precipitation, so the long-term capacity is limited by the mass and the characteristics of soil in contact with the wastewater. Removals are related also to the residence time of the wastewater in the soil, the travel distance, and other climatic and operating conditions.

Rapid-infiltration system design

The design of the annual hydraulic loading rate of a rapid-infiltration system is based on the permeability of the soil or the effective hydraulic conductivity of the soil media that the wastewater infiltrates. The rate is expressed as follows (Equation 6.15) (Metcalf and Eddy, Inc., 1991):

where:

Lw = hydraulic loading rate, ft/yr or cm/yr

IR = infiltration rate, in/h, cm/h

OD = number of operating days per year, day/yr

F = application factor (10-15% of the minimum measured infiltration rate if basin infiltration is used; 4-10% of the conductivity of the most restrictive soil layer if vertical hydraulic conductivity measurements) C''' = conversion factor = 2 for imperial units and 24 for metric units

Application of a rapid-infiltration system to wastewater treatment is not continuous but in cycle; the purpose of nonapplication periods (dry periods) is to allow the land to reaerate and decompose the accumulated organic matters in the soil. The application rate thus can be calculated based on the hydraulic rate, operating cycle time, and application period. The land requirement of a rapid-infiltration system is obtained by dividing the annual wastewater flow rate by the design of the annual hydraulic loading rate in Equation 6.15.

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