Design

The maximum operating level of an anaerobic lagoon is a volume requirement plus a depth requirement. The volume requirement is the sum of the following volumes:

• Minimum treatment volume, ft3 (MTV)

• Manure volume, wastewater volume, and clean water, ft3 (WV)

The depth requirement is the normal precipitation less evaporation on the lagoon surface.

Polluted runoff from a watershed must not be included in a lagoon unless a defensible estimate of the volatile solid loading can be made. Runoff from a watershed, such as a feedlot, is not included in a lagoon because loading would only result during storm events and because the magnitude of the loading would be difficult, if not impossible, to estimate. As a result, the lagoon would be shocked with an overload of volatile solids.

If an automatic outflow device, pipe, or spillway is used, it must be placed at a height above the maximum operating level to accommodate the 25-year, 24-hour storm precipitation on the lagoon surface. This depth added to the maximum operating level of the lagoon establishes the level of the required volume or the outflow device, pipe, or spillway. A minimum of 1 foot of freeboard is provided above the outflow and establishes the top of the embankment. Should state regulation preclude the use of an outflow device, pipe, or spillway or if for some other reason the lagoon will not have these, the minimum freeboard is 1 foot above the top of the required volume.

The combination of these volumes and depths is illustrated in figure 10-21. The terms and derivation are explained in the following paragraphs.

Anaerobic waste treatment lagoons are designed on the basis of volatile solids loading rate (VSLR) per 1,000 cubic feet. Volatile solids represent the amount of solid material in wastes that will decompose as opposed to the mineral (inert) fraction. The rate of solids decomposition in anaerobic lagoons is a function of temperature; therefore, the acceptable VSLR varies from one location to another. Figure 10-22 indicates the maximum VSLR's for the United States. If odors need to be minimized, VSLR should be reduced by 25 to 50 percent.

The minimum treatment volume (MTV) represents the volume needed to maintain sustainable biological activity. The minimum treatment volume for VS can be determined using equation 10-4.

TVS VSLR

where:

MTV = Minimum treatment volume, ft3

TVS = Total daily volatile solids loading (from all sources), lb/day VSLR = Volatile solids loading rate, lb/1,000 ft3/day (from fig. 10-22)

Daily volatile solids production for various wastes can be determined using tables in chapter 4. If feed spillage exceeds 5 percent, VSP should be increased by 4 percent for each additional 1 percent spillage.

Waste volume (WV) should reflect the actual volume of manure, wastewater, flush water that will not be recycled, and clean dilution water added to the lagoon during the treatment period. The treatment period is either the detention time required to obtain the desired reduction of pollution potential of the waste or the time between land application events, whichever is longer. State regulations may govern the minimum detention time. Generally, the maximum time between land application events determines the treatment period because this time generally exceeds the detention time required.

where:

WV = Waste volume for treatment period, ft3 TVM = Total volume of manure for treatment period, ft3

TWW = Total volume of wastewater for treatment period, ft3

CW = Clean water added during treatment period, ft3

In the absence of site-specific data, values in chapter 4 may be used to make estimates of the volumes.

As the manure is decomposed in the anaerobic lagoon only part of the total solids (TS) is reduced. Some of the TS is mineral material that will not decompose, and some of the VS require a long time to decompose. These materials, referred to as sludge, gradually accumulate in the lagoon. To maintain the minimum treatment volume (MTV), the volume of sludge accumulation over the period of time between sludge removal

Figure 1O-21 Anaerobic lagoon cross section f

Max. operating level

Required volume

Freeboard (1.0 minimum)

Depth of 25-year, 24-hour storm event on lagoon surface

Depth of normal precipitation less evaporation on the lagoon surface accumulated during the treatment period

Volume of manure, wastewater, and clean water accumulated (WSV)

during the treatment period

Minimum treatment volume

Volume of accumulated sludge for period between sludge removal events (SV)

Depth of 25-year, 24-hour storm event on lagoon surface

Crest of spillway or other outflow device (where permissible)

Crest of spillway or other outflow device (where permissible)

Note: The minimum treatment volume for an anaerobic waste treatment lagoon is based on volatile solids.

CAUTION SHOULD BE USED IN INTERPOLATING THIS MAP IN MOUNTAINOUS AREAS. LINES HAVE BEEN SMOOTHED IN MOUNTAINOUS AREAS OF THE WESTERN STATES.

SOURCE:

NATIONAL WEATHER SERVICE PUBLICATIONS. MAP PREPARED USI AUTOMATED MAP CONSTRUCTION. NATIONAL CARTOGRAPHY GIS CENTER, FORT WORTH, TEXAS, 1995

REVISED JUNE 1995, 1005008

,2

3

CAUTION SHOULD BE USED IN INTERPOLATING THIS MAP IN MOUNTAINOUS AREAS. LINES HAVE BEEN SMOOTHED IN MOUNTAINOUS AREAS OF THE WESTERN STATES.

SOURCE:

NATIONAL WEATHER SERVICE PUBLICATIONS. MAP PREPARED USI AUTOMATED MAP CONSTRUCTION. NATIONAL CARTOGRAPHY GIS CENTER, FORT WORTH, TEXAS, 1995

REVISED JUNE 1995, 1005008

o era

rt-y

oo co must be considered. Lagoons are commonly designed for a 15- to 20-year sludge accumulation period. The sludge volume (SV) can be determined using equation 10-6.

where:

SV = Sludge volume (ft3) AU = Number of 1,000-pound animal units T = Sludge accumulation time (years) TS = Total solids production per animal unit per day (lb/AU/day) SAR = Sludge accumulation ratio (ft3/lb TS)

Total solids values can be obtained from the tables in chapter 4. Sludge accumulation ratios should be taken from table 10-4. An SAR is not available for beef, but it can be assumed to be similar to that for dairy cattle.

The lagoon volume requirements are for accommodation of the minimum treatment volume, the sludge volume, and the waste volume for the treatment period. This is expressed in equation 10-7.

where:

LV = Lagoon volume requirement, ft3 MTV = Minimum treatment volume, ft3 (see equation 10-4)

SV = Sludge volume accumulation for period between sludge removal events, ft3 (see equation 10-6) WV = Waste volume for treatment period, ft3 (see equation 10-5)

In addition to the lagoon volume requirement (LV), a provision must be made for depth to accommodate the normal precipitation less evaporation on the lagoon surface; the 25-year, 24-hour storm precipitation; the depth required to operate the emergency outflow; and freeboard. Normal precipitation on the lagoon surface is based on the critical treatment period that produces the maximum depth. This depth can be offset to some degree by evaporation losses on the lagoon surface. This offset varies, according to the climate of the region, from a partial amount of the precipitation to an amount in excess of the precipitation. Precipitation and evaporation can be determined from local climate data.

The minimum acceptable depth for anaerobic lagoons is 6 feet, but in colder climates at least 10 feet is recommended to assure proper operation and odor control.

The design height of an embankment for a lagoon should be increased by the amount needed to ensure that the design elevation is maintained after settlement. This increase should not be less than 5 percent of the design fill height. The minimum top width of the lagoon should be as shown in table 10-5, although a width of 8 feet and less is difficult to construct.

The combined side slopes of the settled embankment should not be less than 5 to 1 (horizontal to vertical). The inside slopes can vary from 1 to 1 for excavated slopes to 3 to 1 or flatter where embankments are used. Construction technique and soil type must also be considered. In some situations a steep slope may be used below the design liquid level, while a flatter slope is used above the liquid level to facilitate maintenance

Table 10-4 Sludge accumulation ratios (Barth 1985)

Table 10-5 Minimum top width for lagoon embankments (USDA 1984, Waste...)

Animal type

Maximum height of embankment, ft Top width, ft

Poultry 10 or less 6

Layers 0.0295 11-14 8

Pullets 0.0455 15-19 10

20-24 12

Swine 0.0485 25-34 14

35 or more 15

Dairy cattle 0.0729 -

and bank stabilization. The minimum elevation of the top of the settled embankment should be 1 foot above the maximum design water surface in the lagoon.

A lagoon should be constructed to avoid seepage and potential ground water pollution. Care in site selection, soils investigation, and design can minimize the potential for these problems. In cases where the lagoon needs to be sealed, the techniques discussed in Appendix 10D, Geotechnical design and construction guidelines for waste impoundment liners, can be used. Also refer to Chapter 7, Geology and Ground Water Considerations, for more information on site evaluation, investigations, and testing. Figure 10-23 shows a two lagoon systems.

If overtopping can cause embankment failure, an emergency spillway or overflow pipe should be provided. A lagoon can have an overflow to maintain a constant liquid level if the overflow liquid is stored in a waste storage pond or otherwise properly managed. The inlet to a lagoon should be protected from freezing. This can be accomplished by using an open channel that can be cleaned out or by locating the inlet pipe below the freezing level in the lagoon. Because of possible blockages, access to the inlet pipe is needed. Venting inlet pipes prevents backflow of lagoon gases into the animal production facilities.

Sludge removal is an important consideration in the design. This can be accomplished by agitating the lagoon and pumping out the mixed sludge or by using a drag-line for removing floating or settled sludge. Some pumps can remove sludge, but not deposited rocks, sand, or grit. The sludge removal technique should be considered when determining lagoon surface dimensions. Many agitation pumps have an effective radius of 75 to 100 feet. Draglines may only reach 30 to 50 feet into the lagoon.

Figure 10-23 Anaerobic lagoon recycle systems

Figure 10-23 Anaerobic lagoon recycle systems

Gutter

Recycle pipe Recycle pump

Gutter outlet

First lagoon

Gutter outlet

First lagoon

Recycle pipe Recycle pump

Second lagoon

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