The management of the solids and concentrated contaminants removed by treatment has been and continues to be one of the most difficult and expensive problems in the field of wastewater engineering. Wastewater solids are organic products that can be used beneficially after stabilization by processes such as anaerobic digestion and com- posting.
BIOSOLIDS AND RESIDUALS MANAGEMENT
The management of the solids and concentrated contaminants removed by treatment has been and continues to be one of the most difficult and expensive problems in the field of wastewater engineering. Wastewater solids are organic products that can be used beneficially after stabilization by processes such as anaerobic digestion and com- posting. With the advent of regulations that encourage biosolids use, significant efforts have been directed to producing a “clean sludge” that meets heavy metals and pathogen requirements and is suitable for land application. Regulations for Class B biosolids call for reduced density in pathogenic bacteria and enteric viruses, but not to the levels of Class A biosolids. Further, the application of Class B biosolids to land is strictly regulated, and distribution for home use is prohibited.
Other treatment plant residuals such as grit and screenings have to be rendered suit- able for disposal, customarily in landfills. Landfills usually require some form of dewatering to limit moisture content. With the increased use of membranes, especially in wastewater reuse applications, a new type of residual, brine concentrate, requires further processing and disposal. Solar evaporation ponds and discharge to a saltwater environment are only viable in communities where suitable and environmental geographic conditions prevail; brine concentration and residuals solidification are generally too complex and costly to implement.
Treatment technologies for solids processing have focused on traditional methods such as thickening, stabilization, dewatering, and drying. Evolution in the technologies has not occurred as rapidly as in liquid treatment processes, but some significant improvements have occurred. Centrifuges that produce a sludge cake with higher solids content, egg-shaped digesters that improve operation, and dryers that minimize water content are just a few examples of products that have come into use in recent years. These developments are largely driven by the need to produce biosolids that are clean, have less volume, and can be used beneficially.
Landfills still continue to be used extensively for the disposal of treatment plant solids, either in sludge-only mono fills or with municipal solid waste. The number and capacity of landfills, however, have been reduced, and new landfill locations that meet public and regulatory acceptance and economic requirements are increasingly difficult to find. Incineration of solids by large municipalities continues to be practiced, but incineration operation and emission control is subject to greater regulatory restrictions and adverse public scrutiny. Alternatives to landfills and incineration include land application of liquid or dried biosolids and composting for distribution and marketing. Land application of biosolids is used extensively to reclaim marginal land for productive uses and to utilize nutrient content in the biosolids. Composting, although a more
Expensive alternative is a means of stabilizing and distributing biosolids for use as a soil amendment. Alkaline stabilization of biosolids for land application is also used but to a lesser extent.
New Directions and Concerns
Over the last 30 years, the principal focus in wastewater engineering has been on improving the quality of treated effluent through the construction of secondary and advanced wastewater treatment plants. With improved treatment methods, higher levels of treatment must be provided not only for conventional wastewater constituents but also for the removal of specific compounds such as nutrients and heavy metals. A by-product of these efforts has been the increased generation of solids and biosolids per person served by a municipal wastewater system. In many cases, the increase in solids production clearly taxes the capacity of existing solids processing and disposal methods.
In addition to the shear volume of solids that has to be handled and processed, management options continue to be reduced through stricter regulations. Limitations that affect options are:
(1) landfill sites are becoming more difficult to find and have per- mitted, (2) air emissions from incinerators are more closely regulated, and (3) new requirements for the land application of biosolids have been instituted. In large urban areas, haul distances to landfill or land application sites have significantly affected the cost of solids processing and disposal. Few new incinerators are being planned because of difficulties in finding suitable sites and obtaining permits. Emission control regulations of the Clean Air Act also require the installation of complex and expensive pollution control equipment.
More communities are looking toward (1) producing Class A biosolids to improve beneficial reuse opportunities or (2) implementing a form of volume reduction, thus lessening the requirements for disposal. The issue—“are Class A biosolids clean—will be of ongoing concern to the public. The continuing search for better methods of solids processing, disposal, and reuse will remain as one of the highest priorities in the future. Additionally, developing meaningful dialogue with the public about health and environmental effects will continue to be very important.