NC State Extension Publications


Municipal sludges are the natural end products of a microbial food chain in the wastewater treatment process. Microbes feed on organic components of waste until they can no longer derive energy from it. At this point, sludge consists of mostly cellular material and stable degradation products that are considered safe for application to agricultural or forest lands.

If properly managed, land application is an excellent way to dispose of sludge. Waste can be applied at rates to meet crop nutrient requirements without harming the environment. Both the waste generator and the crop producer benefit from this recycling system. Humans and animals are natural waste generators, and land application makes it possible to recover the valuable components of waste as a usable resource.

decorative illustration of soil


Sludges contain many nutrients nec­essary for plant growth and develop­ment (Table 1) and organic matter that can improve the soil tilth. Ap­plying sludge in accordance with the nutrient requirements of the crop and USEP A guidelines poses little risk to the environment or public health. Sludge is often provided without charge. Its use can therefore reduce fertilizer bills and in­crease the profitability of crops. When sludges are disposed of in other ways, the valuable nutrients they contain cannot be recovered. Concerns about applying sludges to land include the potential for ap­plying too much or too little of each nutrient, the presence of toxic con­stituents, and problems with odors or insects. Sludges contain nutrients that are beneficial to plants, but heavy metals or other potentially toxic substances may also be pres­ent. These substances must be re­duced or confined to levels that are considered safe for both agricultural and forest crops and soils.

Table 1. Median Concentrations of Nutrients and Heavy Metals in Sludges
Metal Parts Per Million
Nitrogen* 2.6
Phosphorus* 1.6
Potassium* 0.2
Lead 335.0
Zinc* 1,750.0
Copper* 475.0
Nickel 37.0
Cadmium 11.0
Chromium 380.0
Mercury 5.0

*Nutrients essential for plant growth.

Source: Agricultural Use of Munidpal and Industrial Sludges in the Southern United States. L. D. King. Southern Cooperative Series Bulletin 314, North Garolina State University.

Application Sites

Sludges can be applied successfully on agricultural land throughout the state. Generally, soils must be rea­sonably deep, well drained, and ag­riculturally or silviculturally produc­tive. They must meet required buffer or setback restrictions. Land­application programs have been suc­cessful in each of the geographic re­gions of the state.

Sludges can be applied legally to either privately or publicly owned land. Both sites are subject to pre­approval by the North Carolina State Department of Environment, Health, and Natural Resources (NCDEHNR). The Department's Division of Environmental Manage­ment (DEM) must issue a permit before sludge can be applied. Before issuing a permit, a member of the division evaluates each site to verify its suitability for waste applications.

Land-applied sludge programs must be managed in accordance with requirements established by the United States Environmental Protec­tion Agency (USEPA) and the De­partment of Environment, Health, and Natural Resources. For more information, see Bulletin 314, Agri­cultural Use of Municipal and In­dustrial Sludges in the Southern United States, and the US EPA De­sign Manual for Land Application of Sludge. Programs described in these publications are designed so that:

  • Valuable agricultural nutrients can be returned to the land;
  • Groundwater and surface water quality are maintained;
  • No significant health or nui­sance problems occur;
  • Future use of sludge-treated land for reasonable activities is not re­stricted.

Obtaining a Permit

The waste producer is responsible for obtaining all necessary permits to apply waste. Permit applications must include an analysis of the waste's nutrient and metal levels, potential toxicity, and liquid and solids content. If the waste analysis indicates that land application is possible, the waste producer must identify suitable sites. This process requires obtaining topographic, lo­cation, and soil maps of each site; preparing descriptions of soil pro­files; and developing a site-manage­ment and monitoring plan to opti­mize waste-management activities and ensure that environmental qual­ity and public health are protected. After the land application permit is issued, the waste producer is respon­sible for the continued operation and monitoring of the facility. Typi­cally, waste, soil, and vegetation must be sampled; groundwater and surface water may also be moni­tored.

Black and white photo of truck with a tank and attachment for injecting sludge into soil

Figure 1. Application of municipal sludge to agricultural lands.

Black and white photo of damaged pasture

Figure 2. Damage caused by application of municipal sludges.

Health and Environmental Concerns

Skip to Health and Environmental Concerns

Disease-Causing Organisms

Only sludges treated by digestion or chemical stabilization to reduce pathogen levels and the potential for disease transmission can be applied to land. Land application further aids in destroying pathogens by ex­posing them to sunlight, the soil en­vironment, and drastic temperature changes. Sludge-application sites are restricted to general public ac­cess for 12 months after the sludge has been applied and three months for grazing animals. Only crops used for animal feed can be grown on sludge-application sites and the permit requires an 18-month lag be­tween sludge application and growth of crops for human consumption.

Heavy Metals

Lead, zinc, copper, nickel, cad­mium, chromium, and mercury are the heavy metals sometimes found in sludge. The total quantity of a heavy metal that can be applied to agricultural or forest land is regu­lated by guidelines from the USEP A and from the Department of Envi­ronment, Health, and Natural Resources. These maximum accumu­lative metal levels are conserva­tively calculated both to protect against metal toxicities in crops and to prevent the movement of heavy metals into the food chain.

Federal and state regulations re­quire that the soil pH be adjusted to 6.5 or more before sludge is applied. This requirement is an additional safeguard that limits metal uptake by plants and promotes optimum crop yields in most soils.

Water Quality

Both surface water and groundwater must be protected in a land-applica­tion program. Nitrogen and phos­phorus are the primary water con­taminates from sludges. Both nutri­ents are necessary for plant growth and can be controlled in an environ­mentally sound manner. Surface waters can be protected by using conservation practices that reduce erosion and prevent the movement of sediments and accompanying nu­trients from the site of application to ponds, lakes, or streams. Ground­water contamination by nitrogen may occur if the nitrogen applied in sludge is greater than the crop re­quires.

The Department of Environment, Health, and Natural Resources in North Carolina restricts sludge ap­plication to sites where surface run­off is minimized and restricted from reaching surface water bodies, drainage ditches, and other im­poundments. Further, application within l 00 feet of wells is prohib­ited to reduce the potential for waste constituents to move from the soil into groundwater.

Insects and Odors

Sludge treatment not only reduces pathogen concentrations but mini­mizes odors by reducing the amount of biodegradable organic material in the sludge. Land application of treated sludge at rates suitable for agriculture or silvaculture normally does not cause problems with odors, insect vectors, or other nuisances. Odor and insect problems generally occur when raw, unstabilized wastes are applied without regard for per­mit requirements.

Sludges may be applied to the soil surface or injected into agricul­tural or forest land. Surface-applied sludge may be disked into the soil, (Figure 1), although incorporation is not required. Injection of sludge is generally the best method for retain­ing nutrients on the site. It mini­mizes the potential for insect and odor problems and may be more aesthetically desirable in a sensitive environmental situation. However, injection of sludges on established pasture sites may damage the pas­ture, (Figure 2), and landowners may therefore not permit this prac­tice. Surface application of sludge has been widely practiced through­out North Carolina and the United States.


Skip to Conclusion

Proper land application of municipal sludges can protect public health, maintain or improve environmental quality, and encourage the benefi­cial use of wastes. Land application of sludges must be permitted by the North Carolina State Department of Environment, Health, and Natural Resources in compliance with state and federal regulations.

Prepared by:
A R. Rubin, Extension Agricultural Engineering Specialist

L. M. Safley, Professor of Biological and Agricultural Engineering

J.P. Zublena, Specialist in Charge, Soil Science Extension

7,000 copies of this public document were printed at a cost of $750, or $.11 per copy.


Associate Dean, CALS and Director, NC Cooperative Extension Service
Extension Administration

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Publication date: Jan. 1, 1991
Revised: June 5, 2024

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