Water Protection Program fact sheet
Division of Environmental Quality Director: Ed Galbraith


Filter backwash lagoons are holding areas for sludge waste created during the drinking water treatment process and are generally permitted by the Missouri Department of Natural Resources under Missouri permit MO-G64. When a water system plans to close a filter backwash lagoon, it is required to submit to the department a closure plan for approval prior to initiating closure activities. The closure plan should detail how the water system plans to dewater the lagoon, remove and dispose of sludge and demolish the lagoon structure.

In accordance with 10 CSR 20-6.010(12)(A), persons who cease operation or plan to cease operation of waste, wastewater and sludge handling and treatment facilities shall close the facilities in accordance with a closure plan approved by the department.

Closure plan activities should be completed to prevent any unlawful pollutants from entering waters of the state and it should create a permanent record of the date and time, volume and methods of removal and disposal of such substances maintained by the water system. Before the permit can be terminated, closure is usually conducted in three phases:

This fact sheet outlines the elements of the closure plan to be submitted to the department.

Absolutely no closure activities, including draining, dismantling or construction, can begin before the closure plan is approved and required permits are issued.

Technical Requirements

Lagoon Dewatering

The wastewater may be discharged through the permitted outfall pipe of the lagoon, in accordance with permit conditions and effluent limitations. Monitoring of the effluent’s permitted parameters should be done at a minimum of once per day during the dewatering phase to ensure permit limits are being met. Great care should be taken by the operator to cease discharge before reaching the sludge blanket, so that no sludge is released to the receiving waters. The remaining residuals may be left inside the basin for drying, or could be placed in a water-tight roll-off container or open top tank. Under no circumstances should residual sludge be removed from the permitted basin and placed directly on the ground for drying.

If wastewater is to be removed from the basin, three options are available for dewatering:

Irrigation requirements

A separate permit is not required for irrigation of overlying lagoon water. Prior to application, the permittee must obtain approval of the method and location of the application. The irrigation plan should be included with the closure plan for the facility and the following minimum restrictions must be included:

Sludge Disposal

Several options are available for disposal of the sludge. Residual sludge may be removed from the basin and taken to a permitted wastewater treatment facility or sanitary landfill, or land applied. Only after the material testing results are available can land application of the sludge or leaving material in place inside the basin be considered. Land application of sludge must be done in accordance with the conditions of the permit, at agronomic rates, using appropriate best management practices as needed.

Backwash basin sludge may be hauled to a sanitary landfill, land applied, left in place or directed through the wastewater treatment facility.

For sludge land applied, the operator or engineer must meet agronomic loading rates (see guidance calculations to determine loading rates). Land application shall not exceed the most restrictive of the following criteria:

For sludge left in place and incorporated into the lagoon berms the operator/engineer must meet agronomic loading rates (see guidance calculations on page 1 to determine loading rates). Sludge must be mixed with native soil at a ratio of no less than 1:1 sludge to soil and shall not exceed the most restrictive of the following criteria:

First, quantification of sludge in the backwash basin is required. To do this the surface area of the basins or area or land application (acres), total sludge volume (gallons), and total weight of wet sludge (tons) must be calculated. Total sludge volume is found by using the current operating sludge depth. A composite sample of sludge must be collected to determine the percentage of solids, which is necessary to calculate total dry weight of backwash basin sludge. The recommended method for composite sampling is to collect and combine seven to 20 individual grab samples of the same volume. The samples should be collected during the same week and from various locations in the sludge layer of the lagoon so as to obtain a composite sample representative of the contents of the entire lagoon. Composite sludge and soil samples shall be tested for the nutrients, metals and other parameters as listed in Table 1 below if the sludge is to be left in-place or land applied. The result of the sludge and soil analyses will be used to determine agronomic loading rates. The following calculations may be used as guidance in quantifying backwash basin sludge and analytes:

Weight of Sludge (tons)


% Solids


Total dry weight of sludge (tons)

Total dry weight of sludge (tons)
Applied surface area (acres)


Total dry tons

Analyte (mg/kg)
453,592 (mg/kg)

907 (kg/ton)
loading (lb/ton)

Total dry tons

Loading (lb/ton)
Total Loading (lbs/acre)

sludge and soil testing requirements chart

For assistance, contact your local regional office.