Biosolids are derived primarily from a combination of primary, secondary, and tertiary sewage sludges. Biosolids do not contain the coarse grit and screenings removed from raw wastewater during preliminary treatment steps or ash generated from incineration of sewage sludge. Another type of biosolids may be derived from domestic septage, the solid or liquid material removed from septic systems, portable toilets, and other systems that receive only domestic sewage. However, since all sludges and biosolids contain potentially harmful cntaminants, beneficial uses must be balanced against acceptable risks for human health and environmental impacts (Beneficial, cornell)
The terms sludge and biosolids are in some cases universally interchangeable, while in some cases there are minor discrepancies between the two definitions. Thus for the sake of this paper we will treat the two definitions as equal and interchangeable.
While in most cases, the composition of the received wastewater is uncontrollable, the makeup of the resultant sludge must be known in order to determine its suitablility for various uses (Prod, cornell). When evaluating the composition of sludge, its physical and chemical properties must be considered. Questions that should be asked are: How much water and solid matter does it contain? How many different metals and nutrients does it have and how much of each? What potentially toxic organic chemicals are contained in the sludge? What pathogen reduction process was used? (Char, cornell).
Typically, sludge contains 1-7% solid material and 93-99% water. (char, cornell). Proper sludge treatment and management are necessary to ensure public and environmental health and safety (char, cornell). The pH of sludge ranges from 4 to 12, and this value can have an affect on the movement of heavy metals, the corrosiveness of the sludge and the survival of the pathogens it contains
Many people, including agricultural scientists and environmentalists are concerned that land application of sludges will result in an increase of pathogenic bacteria, viruses, parasites, chemicals and metals in drinking water reservoirs, aquifers and the food chain (benificial, cornell). While the potential risks may seem to be high in the applications of sludge, the degree of risk depends directly on the initial sludge quality, the way sludges are processed and how the sludges are managed during application (benificial, cornell).
General Overview: Modes of Transport from Field to Human
There are numerous pathways by which humans can come into contact with biosolids or biosolids-derived contaminants. These include direct contact or accidental ingestion, inhalation of biosolids-derived aerosols or dust, ingestion of water (surface waters and groundwater), and consumption of crops grown in biosolids-amended soils or of animals that have fed on crops grown in such soils. In addition, a variety of vectors can transmit pathogens (flies, mosquitos, fleas, rodents, or other animals than can transport the disease either mechanically or by biological processes) from biosolids to humans or intermediate hosts. In order to cause infection, pathogens must gain entrance to the body of the host. Microorganisms cannot penetrate normal, undamaged skin but can pass through mucous membranes, which thus form the most common portals of entry (Sterritt, 1988). These occur at the alimentary, respiratory and genitor-urinary tracts (Silverthorn, 1998). Additional...