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Freshwater Management Series No. 1

Biosolids Management: An Environmentally Sound Approach
for Managing Sewage Treatment Plant Sludge

An Introductory Guide To Decision-Makers

Notes for the Reader

All municipal and industrial wastewater treatment processes generate two primary outputs: a liquid effluent and a residual solid material. The treated liquid is the most visible output and great emphasis is placed on meeting prescribed quality standards to minimise risks to human health and the environment. In contrast, disposal of residual solids is often overlooked and the material is either ignored, disposed of as a hazardous waste, or in some areas of the world, utilised as a valuable commercial and agricultural resource. This inconsistency is due in large part to misunderstandings about the nature of this residual material and its utilisation as an environmentally sound management practice. This introductory guide has been prepared for stakeholders and decision-makers in areas where the management of residual solid materials from sewage treatment plants and the utilisation of biosolids are not fully understood. The information presented is intended to clarify the important issues and provide guidance to decision-makers seeking the best course of action.

It is important for the reader to note that septage from on-site treatment (i.e., septic tanks) and faecal sludge from bucket and vault latrines are not addressed in this guide. Each of these wastes has unique characteristics and management requirements. More information on these types of residual wastes can be obtained by referring to the “International Sourcebook on Environmentally Sound Technologies for Wastewater and Stormwater Management”, which is available through the UNEP International Environmental Technology Centre (IETC).

Biosolids Management: An Issue of Global Importance

In many urban regions of the developing world, municipal and industrial sewage is collected and then used without any treatment for irrigating crops. Farmers receive the value of the nutrients dissolved and suspended in the wastewater but may pay a price in sickness, and in many cases, damage to soil from discharged industrial wastes due to the potential accumulation of significant quantities of heavy metals and other pollutants. Where treatment does occur, residual solids, including the biomass from the treatment process, are often discharged back into a nearby watercourse, thereby posing the risk of contaminating down-stream sources of potable water. When direct discharge is not available, dewatered material may simply be stored on site, contaminating both soil and groundwater. Recognising that access to a reliable supply of safe water is a basic right of citizens, municipal authorities in many developing countries are now in the process of establishing water resource master plans and building new water and wastewater treatment systems. Concerned citizens and community leaders are also beginning to appreciate the importance of finding sustainable solutions for the discharged residual solids, including the beneficial application of biosolids to land.

In northern countries, land application of biosolids as a fertiliser and soil conditioner has been increasingly used over fifty years and this practice is now widespread. However, serious issues can arise in the absence of an appropriate regulatory framework designed to ensure an acceptable level of quality at each critical stage of the biosolids management and utilisation process. Where public officials and private firms fail to maintain the essential elements of a well-regulated biosolids management system the confidence and trust of the local community may be lost. As a consequence, some biosolids utilisation programmes have been discontinued in favour of landfill disposal while others do not receive the local support needed in order to develop the practice. Successful community-based biosolids utilisation requires good planning, stakeholder involvement, the nurturing of local capacity, and the implementation of the necessary enabling systems. Above all, it requires an open and inclusive communication process.

Figure 1
Figure1   Figure 1: Simplified Wastewater Collection and Treatment System

Some Basic Terms

The raw material from which treated wastewater and biosolids are produced is a combination of storm water run-off and sewage discharges from households (including human waste), commercial businesses and industrial plants. This mixed material flows by gravity to a wastewater treatment plant where the combined influent passes through a series of mechanical and biological processes. The term municipal sludge is used to describe the residual solids that are separated and collected during the wastewater treatment process. Biosolids is the term used for the nutrient-rich organic material separated during the wastewater treatment process that, after receiving additional treatment and passing rigorous quality requirements, is used as an agricultural or commercial fertiliser and soil-conditioning material. Figure 1 provides a simplified illustration of wastewater collection and treatment leading to the production of biosolids. When properly prepared and applied, biosolids provide growing plants with important primary nutrients such as Nitrogen, Phosphorus, Potassium, Sulphur, Magnesium and Calcium, and micronutrients such as Copper, Iron and Zinc. Biosolids management is the integrated network of sustainable practices, public information programmes, regulatory requirements, monitoring systems and receptive markets that ensure that biosolids utilisation provides expected benefits without incurring risk to public health or the environment. The terms human waste, municipal sludge and biosolids are sometimes used interchangeably, but in fact, they are all distinctly different materials. The safe and beneficial utilisation of processed biosolids is possible only when a management system is in place and working properly.

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