Effective disinfection and stabilization of sewage sludge prior to land application is essential to not only protect human health, but also to convince the public of its benefits and safety. A basic understanding of the key factors involved in producing a stable biosolid product is a necessary component to ensuring that effective disinfection and stabilization are achieved. Key stressors used to treat sewage sludge, both traditional and some emerging, are discussed including physical, chemical, and biological stressors. Factors that affect a stressor’s effectiveness are included where information is available. Examples of methods that employ each stressor are presented.
Sewage sludge resulting from municipal wastewater treatment must be treated in accord with federal and state requirements prior to being applied to land as a soil amendment, providing conditioning and fertilizing benefits. Such treated sludges are commonly referred to as biosolids. The most current data indicates that land application of biosolids is the most common use/disposal option for sewage sludge at 60% (NRC, 2002). Several factors are expected to continue pushing the use of biosolids on land upwards. These factors include a decreasing pool of use/disposal options to choose from, increasing costs of landfilling, and increasing air controls on incineration. Estimates are that only 0.1% of available agricultural land in the United States has biosolids land applied annually (NRC, 2002). Thus, a tremendous untapped market exists to support such expansion in the use of biosolids.
Despite the expected growth in the land application of biosolids, public acceptance of the practice has lagged behind. Because of these external pressures care must be taken to produce the best possible product. This entails a basic understanding of the key factors involved in producing a stable biosolids product.
Although there has been some debate over the exact definition of stabilization, it generally includes three main parts:
- pathogen reduction or disinfection,
- elimination of offensive odors and a general improvement of aesthetics,
- minimization in the potential for putrification.
The 40 CFR 503, Standards for the Use or Disposal of Sewage Sludge breaking sludge stabilization into two main sets of regulations laid out in Subpart D, Pathogens and Vector Attraction Reduction. This subpart provides alternatives for achieving pathogen reduction in sewage sludge. The pathogen reduction requirements are divided into two levels, Class A and Class B, depending upon how complete the disinfection. In Class B, disinfection is incomplete; indicator organisms (namely, fecal coliform) are reduced by approximately two logs to densities below 1 million per gram of total solids (g TS) dry weight and pathogens are reduced by approximately 10%. Processes which produce a Class B product are called PSRPs or processes which significantly reduce pathogens. In Class A, disinfection is more complete; fecal coliform levels are less than 1,000 MPN/g TS-dry weight and pathogens are reduced below the level of detection. Processes which produce a Class A product are called PFRPs or processes which further reduce pathogens. Many of these disinfection alternatives, regardless of class will simultaneously reduce the attractiveness of a sludge to vectors, such as digestion and alkaline treatment, but some alternatives may require partnering with a separate vector attraction
Vectors are attracted to sewage sludge because it is a possible food source. Typically, they are tipped off to a food source by offensive odors produced by any remaining putrescible materials. Vector attraction reduction is accomplished in two main ways, oxidative treatment or physical barriers in which the sewage is injected under or incorporated into the topsoil. Treatment can be accomplished biologically in which the available food source is consumed or oxidized by microorganisms or chemically/physically in which the environment within the sewage sludge is changed such that microbial activity cannot be supported. Technically speaking, a stable sludge will undergo no further change. Thus, permanent stabilization can only be achieved by a biological vector attraction reduction option. Vector attraction reduction through chemical or physical treatment options, such as lime stabilization and desiccation are temporary, because these options do not remove the underlying cause of putrification and odors, i.e., the food source.
The remainder of this paper will review the key factors or stressors involved in traditional and some emerging disinfection/stabilization methods in contemporary literature. More detailed descriptions of all the traditional methods called out in part 503D can be found in U.S. EPA (2003).