Waste Disposal

Sludge And Septage Land Application

Municipal sludge is the residual of the wastewater treatment process. Wastewater treatment generally accelerates the natural biological process of purification by creating optimum conditions for microbial growth. As the microbes gain mass and die off they separate from the water and settle as sludge. The sludge is then stabilized through anaerobic (oxygen-free), aerobic (oxygenated), thermal or other treatment processes to effectively minimize pathogen densities as well as its attractiveness as a food source for vectors of disease transmission (rodents, birds, insects, etc.). Sludge generally contains substantial levels of nitrogen and organic material as well as phosphorus, potassium, and a combination of micro and macro nutrients. Sludge treatment, quality, final disposition, and general management is regulated by ch. NR 204, Wis. Adm. Code, which was revised effective January 1, 1996 to incorporate federal standards published in 1993. In developing the federal standards, U.S. EPA conducted the most extensive peer-reviewed risk assessment ever undertaken by the agency. Several levels of limits were established through this process on allowable metal concentrations in sludge and cumulative limits on application sites. As a testament to the success of the pretreatment program, more than 98 percent of all municipalities in the state are able to satisfy the more restrictive concentration limits that assure that the cumulative loading limits will never be reached. Wisconsin has kept cumulative loading limits on all application sites since the late 1970s and only a very few sites have even approached 10 percent of the cumulative loading limit.

Wisconsin has been a national leader since the middle 1970s in recycling sludge as fertilizer through application on agricultural land. Of all municipal sludge generated in the state, 99 percent is recycled in this manner, including all facilities within the basin (although one facility does landfill in the winter). Every application site must be approved prior to use. Such approval is based upon many criteria, including site characteristics, slopes, setback from surface waters, residences, wells, public areas, depth to high groundwater or bedrock, and soil permeability. In order for sludge to be land applied, requirements must also be satisfied with respect to: pollutant concentrations, pathogen control, and vector control. In addition, application is limited to the nutrient needs of the crop to be grown considering all sources of nitrogen to be supplied, including animal manure and commercial fertilizer. Crop recommendations are based on soil analyses, which are required at least every four years. The regulations assume the organic nitrogen is slowly released over time and therefore require the use of mineralization rates for the first three years after application, which must be taken into account when setting application rates. Also, if a community water supply has naturally occurring radium-226 at concentrations greater than 2 piC/l, the sludge is monitored for radium and land application is limited to a cumulative loading level of 2 piC/g at each site.

Recognizing that phosphorus levels in sludge must increase as effluent limits are decreased, and that phosphorus in sludge is not poisonous to plants and is generally insoluble, special attention is given to ensure that sludge remains on land and does not runoff to surface water. Therefore the department essentially prohibits land application of sludge on frozen or snow-covered land, by requiring 180 days of sludge storage or other means of eliminating the need to land apply in inclement weather. This requirement, the physical site restrictions and application rate limitations effectively minimize the runoff potential from land application.

Unlike sludge, septage is either the solids or wastewater generated by private on-site wastewater systems and the treatment and final disposition is regulated by ch. NR 113 Wis. Adm. Code. Septage is either taken to a treatment works for further treatment or is directly land applied on approved sites. Site use is based on the same criteria as municipal sludge sites. Pathogen control and vector attraction reduction requirements are imposed on septage if it is to be land applied. Application is limited to the nutrient needs of the crop although in most cases application is limited hydraulically to 39,000 gallons per acre per year. This application rate assumes the crop needs 100 pounds of nitrogen per acre. If the crop needs less than that, the application rate must be reduced. If the crop needs more than that, special permission must be obtained in order to increase the application rate and additional requirements are imposed.

Sanitary Sewers

Miles of sanitary sewers are located in cities, villages and sanitary districts throughout the state. Infiltration of groundwater into sewers has been the subject of investigation because the excess flow can overload the sewage treatment plant. Yet, little attention has been paid to exfiltration, which is the leaking of sewage from sewers into the ground. This an area of growing concern and research.

Private Wastewater Systems

Unsewered communities, where residents live outside the reach of municipal sewer systems, rely on septic tanks, mound systems or holding tanks to dispose of their domestic wastewater. A conventional private wastewater system consists of a septic tank and a soil absorption field.

Household wastes are discharged into an underground water-tight septic tank. Most solids in the wastewater settle to the bottom of the tank and are partially decomposed by bacteria to form sludge. The septic tank effluent flows from the septic tank to the soil absorption field where adsorption and biodegradation, or breakdown of the organic matter, takes place as the waste moves through the soil. If the soil is coarse, the effluent travels quickly and is not held long enough to allow the attachment and breakdown of contaminants and pathogens.

The location of a soil absorption field has important implications for groundwater protection. Locating a soil absorption field in shallow soils can result in groundwater pollution, as the contaminants will move through the shallow soils quickly and reach the upper and lower aquifers. The contamination may result in pollution of potable wells, depending on site-specific geologic features and proximity to the pollution source. Other factors that affect the potential for drinking well contamination include whether the well is located downgradient of the soil absorption field or if the field is poorly designed, sited and maintained. The number of septic system failures can be substantially reduced with greater understanding of the design, operation and maintenance requirements of septic systems. Many homeowners are unaware that their systems require regular maintenance, including inspections and monitoring.

Holding tanks are used when soil depth requirements for conventional septic systems are not available. Holding tanks are very expensive to operate due to pumping frequency and high pumping costs. Although the initial cost of installation may be low compared to other private septic systems, high maintenance and pumping costs may make the use of holding tanks prohibitive. Proper installation and routine pumping is critical for minimizing impacts on groundwater.

Pollutants from septic systems include nitrates, bacteria, viruses, and hazardous materials from household products. These pollutants are described in greater detail later in this section. Even in properly functioning systems, some nitrate is discharged to groundwater. Considering the limitations of the soils in this basin for liquid waste disposal it is important to maintain systems to keep them functioning properly.

DILHR regulates siting, design, installation, and inspection of systems and licensing of installers and evaluators under Wisconsin State Statues 145 and 236, and Wis. Admin. Codes NR 83 and 84. State inspection is required for large-scale systems.

Solid Waste Disposal Sites

Solid waste disposal sites are important potential sources of groundwater pollution. In the past, solid waste was disposed of in excavations, wetlands, sinkholes and low lying areas, or was used as construction fill. The Resource Conservation and Recovery Act (RCRA) of 1976 prohibited this open dumping practice.

Continuous or intermittent contact between refuse and water produces an undesirable liquid called leachate. Landfill leachate is a grossly polluted liquid with high concentrations of dissolved chemicals, high chemical and biological oxygen demand, and hardness. Leachate may contain substances from hazardous materials at the site. The threat to groundwater from waste disposal sites depends on the nature of the leachate, the availability of moisture in contact with the refuse, the type of soil through which the leachate passes, and the hydrogeology of the site. The success of an adequately sited and designed disposal site depends on how leachate production and movement is prevented or minimized.

The pollution threat from old or abandoned landfills varies widely, depending upon what was placed in the landfill and where the landfill is located. Sites containing only domestic wastes and very small amounts of chemicals are at a lower risk than those containing large amounts of chemicals. In addition, sites without a leachate collection system pose a greater risk.

WDNR, as mandated by the 1984 groundwater rule, Wisconsin Act 410, ranks abandoned landfills according to their potential to harm the environment. Each facility is ranked according to the type of waste it contains, the area of disposal and distance to water supply wells, and the physical characteristics of the location such as soils, bedrock and depth to the water table. Site repair or cleanup depends largely on its hazard ranking. Information concerning this ranking is available from the WDNR Bureau of Solid and Hazardous Waste Management.

For more information on groundwater, visit the WDNR - Drinking Water and Groundwater Program