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.

The Pathogen Equivalency Committee has updated the criteria it uses to make recommendations of equivalency on innovative or alternative sludge pathogen reduction processes. To assist new applicants through the equivalency recommendation process the pathogen equivalency committee has rolled out a new website. The Pathogen Equivalency Committee website clearly presents the criteria used for equivalency and provides guidance for developing the proper testing and documentation. The main steps of the equivalency recommendation process are discussed through the introduction of the website materials. A hypothetical case study is presented to demonstrate use of the website and some key points in developing an application for equivalency to a process to further reduce pathogens.

The inactivation of pathogens in biosolids and residuals can be achieved to physical, chemical and biological stressors. The physical stressors include temperature, cavitation and radioactive irradiation (gamma and beta). The chemical stressors are associated with alkaline agents (raising the pH and causing exothermic reactions), acidic agents (lowering the pH and causing exothermic reactions), oxidation and reduction agents (destroying organics and stabilization of uncharged disinfectants) and non-charged disinfectants (required to disinfect helminth eggs and bacterial spores). The biological processes result in the temperature increases greater and result in the production of biocidal by-products such as organic acids, aldehydes and alcohols, which act as disinfectants. In general, biological residual treatment processes such as composting, anaerobic digestion, alkaline stabilization, lagoon storage and air drying, which involve multistressors factors and are generally more effective and economically viable.The understanding of stabilization is moving from the state of science in the 1970s to the 21 st Century. This entails the control of putrescible organics with the potential destabilization of anaerobically digested biosolids. By understanding the role of oxidants in the stabilization processes, the approach to stabilizing biosolids is changing while utilizing the concepts in collection systems. In the more recent studies, the usage of oxidation-reduction potential is very useful in monitoring these systems along with the usage of Pearson soft acids such as chlorine dioxide and ferrate.