Biohazards of Sewage Sludge

Wastewater can come from a variety of sources like homes, businesses, industries, runoff from roads, lawns and fields and is treated by municipal wastewater treatment facilities (Product, cornell). The solid material that remains after the treatment of wastewater is known as sludge (charac, cornell). Sludge can be employed in many different ways, it can be used as a soil additive or growing medium, sent to a composting facility, incinerated or landfilled (Char, cornell).

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 routes of entry are via abrasions or small openings in the skin where local infections such as boils may occur, via wounds from which infection may spread throughout the body by means if the circulatory system, or via animal or insect bites (Sterritt, 1988).

These various routes or pathways of contact can result in either acute or chronic disease

if the exposure is high enough. For pathogens, the primary concern is acute diseases of a short-term duration (i.e., gastroenteritis or flu-like symptoms), while for the various potential chemical contaminants, risks are derived from chronic exposure via ingestion.

Pathogens that may be present in biosolids applied to land pose a disease risk only if there are routes of exposure that deliver an infective dose. The principal means of exposure is through ingestion or inhalation. Absorption through the skin is considered to be a minor route of exposure unless a field worker suffers a cut or other puncture to the skin and is exposed. The degree of harm caused by bacterial toxins varies with the mode of entry into the body.

It seems there is a correlation between the amount of biosolids applied and frequency of contamination. A 1984 study showed that 22.4mg/ha applied to Sassafras sandy loam soil was the upper limit to ensure that groundwater was not contaminated (Higgins, 1984). Doses below 22.4mg/ha of land are acceptable for the provision of nutrients to plants, while anything above diminished the water quality to below American federal standards (Higgens, 1984). Heavy metals and coliforms remained low throughout the experiment, which is also backed by research from health Canada showing that heavy metals and coliforms detected in groundwater are at concentrations lower than ones of concern (HealthCanada, 2000?).

When biosolids are applied to the land, pathogens that may be present in the biosolids can be deposited on plants, either directly from application operations or indirectly by vectors. Virus transport from soil to plants has been suggested as a possible route of exposure, but no definitive research has shown this to occur (Straub et al.1993). Planting restrictions are applied on biosolids-amended fields to ensure that contamination of plants is minimized until die-off of any residual pathogens have occurred and risks are reduced.

Potential bacterial and viral pathogens carried by animals that could be contracted by humans include tuberculosis, salmonella, lysteria, campylobacter, rotovirus, and