SLUDGE RE-USE AND DISPOSAL
Because sludge consists of nutrients, organic molecules and trace metals that are needed by plants for growth, it makes an excellent soil conditioner. In fact, in countries such as China, various forms of sludge have been used as a soil conditioner for thousands of years. The general public often expresses concern when the subject of using wastewater treatment sludge as a soil amendment comes up. Many of the public’s concerns have a basis in fact and must be addressed. However, some of the concerns are simply “knee-jerk” reactions that can be dispelled with good information. As the operator of a wastewater treatment plant, you may be involved in setting up or running a sludge beneficial use program. If you are, you have two responsibilities;
(1) Follow all of the state and federal regulations carefully, and
(2) provide a product that is as consistently safe as possible and ensure that it is properly used.
Sludge that has been treated for beneficial use is generally referred to as Biosolids. When applied to soil in the correct amounts, biosolids can greatly improve the soil’s ability to retain water as well as improve the aeration of the soil.
If it is not practical to beneficially use all or any of the sludge generated in a wastewater treatment facility, some safe and economical form of disposal will have to be employed. The alternatives for the beneficial use of biosolids generally involve some form of land application (either distributed in bulk or in bags). Sludge disposal is typically done by surface disposal or landfilling, although incineration is permissible. Be aware that state and federal laws are in place to regulate the use and disposal of all sludge generated by wastewater treatment facilities. FAILURE TO ABIDE BY THESE LAWS CAN LEAD TO CIVIL AND EVEN CRIMINAL PROSECUTION.
When dealing with sludge issues you may have to seek permits or approval from the federal Environmental Protection Agency and possibly State run Offices of Water Quality. Although it is not practical to outline all of the regulations surrounding each of the sludge use and disposal practices, the following is an overview of the practices most common to our state.
LAND APPLICATION OF BIOSOLIDS
Land application of biosolids is a beneficial use practice that involves applying sludge to vegetated land at or near the vegetation’s agronomic uptake rate. In addition to supplying the vegetation with nutrients, the condition of the soil is improved. Three major areas of concern exist with regard to sludge that is land applied. These are;
(1) pathogenic contamination, which is generally measured with the indicator organism Fecal Coliform
(2) vector attraction reduction (VAR) which prevents animals from being attracted to the sludge and
(3) toxins or potential toxins such as heavy metals, PCBs and nitrogen compounds which could contaminate the natural environment.
The law that governs the disposal and use of wastewater sludge is Title 40 of the Code of Federal Regulations (CFR), part 503 (commonly referred to as the 503 sludge regulations). The 503 sludge regulations set forth treatment techniques that are to be employed to reduce the level of pathogens in sludge before it can be land applied. Two tiers of pathogen reduction through treatment exist: Class “A” and Class “B”. Class “A” sludge is of high quality and can generally be sold or given away to the general public if it also meets the VAR and heavy metals requirements of part 503. Class “B” sludge is of lesser quality, but can still be land applied in bulk when specific management practices are followed and the sludge meets the VAR and heavy metal requirements of part 503. Approval or even a permit may also be necessary to land apply Class “B” sludge.
One of the most common Class “A” treatment options is the composting process. Composting is a thermal aerobic biological process. Generally speaking, sludge is placed in a pile, called a windrow, along with wood chips or other mulched green waste. Bacteria and other organisms in the pile begin to digest the sludge and green waste. The pile is mechanically aerated or turned to provide oxygen so that the aerobic organisms remain active. The activity of the organisms creates substantial heat and high temperatures in the compost pile will result. To meet Class “A” standards, a temperature over 55º C (131º F) must be maintained for fifteen days with five complete pile turnings during that time. Careful control of the mixture and moisture content are required to achieve such high temperatures through aerobic organism activity alone. The high temperature and digestion greatly lower the number of pathogenic organisms remaining in the finished product.
SURFACE DISPOSAL OF SLUDGE
If sludge cannot be beneficially used, it must be disposed of. Surface disposal is a common method of disposing of large amounts of sludge, both in liquid and solid forms. Surface disposal involves applying sludge to the land surface well above the agronomic uptake rate of any vegetation that may be present. The sludge can be injected as a liquid 1–3 ft below the surface or spread on the land as a solid and then plowed in to incorporate it into the soil. The 503 sludge regulations have specific requirements for surface disposal operations. These requirements mainly involve the maximum amount of heavy metals and other toxins that can be applied to the land (for all time) as well as VAR options, site restrictions and management practices that must be followed. A permit is required for all sludge surface disposal sites and ground water monitoring will most likely be required as part of the permit. Although it is not generally considered as environmentally friendly of an option as land application, surface disposal can be a safe and economical alternative when performed properly.
LANDFILLING OF SLUDGE
Landfilling sludge is one of the least desirable options for sludge disposal; however, it is a common practice. It is undesirable because valuable landfill space is wasted on material that could be incorporated back in to the soil as a benefit. Landfilling of sludge should only be practiced when economics or poor sludge quality make it practical.