UNDERSTANDING THE WASTEWATER TREATMENT PLANT IS THE KEY TO CO-DISPOSAL OF WATER TREATMENT PLANT RESIDUALS

Management of residuals is an important component of water and wastewater treatment. These residuals are often disposed separately for a number of reasons such as different entities managing water and wastewater treatment in many areas, significantly different characteristics of water and wastewater residuals (i.e. chemical residuals vs. biosolids), significant differences in impact to land application sites, differences in the quantities of residuals, and cost considerations.Water treatment plant (WTP) residuals normally consist of chemical residuals from coagulation and settling or coagulation and filtration steps. Alum and ferric chloride are frequently used as coagulants (forming a metal hydroxide precipitate), while lime may be used in the softening process. Coagulant WTP residuals quantities can be just a fraction of those produced at a wastewater treatment plant (WWTP) on a per mgd of water processed basis, and may be landfilled, land-applied, or discharged to the WWTP. Sometimes, the WTP hydraulic capacity can be much larger than the WWTP receiving the residuals for disposal. Softening residuals production can exceed biosolids production for comparable flows. WWTP residuals may consist of unstabilized primary and biological residuals, or stabilized biosolids from digestion, alkaline stabilization, composting or drying processes. WWTP biosolids are landfilled, incinerated, or land-applied in accordance with 40 CFR Part 503 Regulations.Despite the differences, there are opportunities for co-management of residuals. In some areas, disposal of WTP residuals to the WWTP can be an attractive comanagement option. WTP residuals can be discharged to the wastewater collection system or blended with wastewater or sludge at a specific point in the WWTP process. This paper focuses on the impacts of WTP residuals on WWTP processes, and includes a series of case studies of municipalities that are co-managing their residuals.