Biosolids Sampling and Handling Approaches for Microbial Analyses

The protection of human health and the environment while making the use of biosolids products more practical is a joint goal of utilities and regulatory agencies. Assuring the safety of biosolids is paramount to gaining wide public acceptance of this waste-to-beneficial use product. Making effective decisions about the safety of biosolids use relies on quality data. Satisfying maximum indicator or pathogen permit levels in conjunction with meeting process pathogen reduction criteria are critical elements of a multiple-barrier approach to assure biosolids safety. Much emphasis and research has been placed on the precision and accuracy of the laboratory methods used to quantify potentially hazardous constituents (i.e. pathogens and metals) in biosolids. However, the errors and variability in the data caused by differences in sample collection and handling can be as significant as those from the laboratory methods, a source of variability that is often overlooked. Some existing regulatory guidance does include requirements for the number of samples, sampling schedules, and sample handling and analyses. These guidelines appear to offer little detail as to actual practices in the field such as sampling techniques, sample locations within tanks and piles, or how to obtain statistically representative samples. This work (WERF project 04-HHE-7) examines several variables affecting the ability to obtain representative sludge and biosolids samples for regulatory compliance with microbial testing requirements. This particular project focused on liquid, dewatered cake, and composted biosolids materials as well as recommended EPA biosolids analysis methods for fecal coliforms (Methods 1680 and 1681), Salmonella spp. (Method 1682), and the method for coliphage in water (Method 1602) modified for biosolids slurries. A combination of literature review, guidance document review, controlled laboratory experiments, and statistical analyses examined the effects of numbers of samples analyzed, sample holding times and temperatures, and approaches to achieve statistical reliability. This work focuses on the results of the controlled laboratory experimentation and statistical analyses.