SIMPLE, EFFECTIVE, LOW COST AND INNOVATIVE BIOSOLIDS DRYING BED DEWATERING/PROCESSING SYSTEM
Abstract:It has been estimated that solids handling systems may account for as much as 25–50% of the total capital and annual costs of a wastewater treatment plant (WWTP). Solids dewatering has been a typical bottleneck at many WWTPs due to operator and maintenance intensive equipment. Simple and effective solids dewatering processes are needed to reduce operator attention and cost throughout the United States. This paper summarizes the steps that were taken to implement a simple, low cost, effective and innovative dewatering process for the Williams Monaco Wastewater Treatment Plant (MWWTP) for the South Adams County Water and Sanitation District (SACWSD), Colorado. This paper focuses on the polymer assisted Deskins “Quick Dry” drying beds (filter bed) portion of the biosolids management system and the process performance results. The air drying/storage process is the final step in the system and is anticipated to be installed next year.
A Biosolids Management Study was completed in 1997 by Rothberg, Tamburini and Winsor (RTW). A detailed analysis of ten (10) Class A and Class B biosolids management alternatives was performed to handle the projected solids production in 2016. The ten alternatives were screened to some preferred biosolids management alternatives using cost and non-monetary selection criteria. The selected alternative included retrofitting the conventional sand drying beds to polymer assisted Deskins “Quick Dry” sand drying beds (filter beds), followed by Class A air drying.
A recent improvement in sand bed drying technology developed by Deskins Company, called “Quick Dry Filter Beds,” has greatly increased the loading capacity of conventional sand beds by a factor of three or four times. Polymer assisted filter beds enhance the rate of water draining from solids. As a result, the solids dry more quickly and may be removed and replaced more often, thus allowing more material to be dewatered. Gravel drainage trenches and perforated piping similar to what is used for conventional sand beds for filtrate collection are utilized, but the sand and gravel media is layered and supported by inexpensive polyethylene cells. The honeycomb cells provide support for the filter media that allows traffic by small biosolids retrieving equipment, without media compaction or the wasted space of concrete pavers. The drainage properties of the “Quick-Dry” filter beds allow greater application rates and shorter drying cycle times.
The “Quick Dry Filter Bed” system also employs a simple in-line polymer feed and rapid mix flocculation system to improve dewatering efficiency. The SACWSD project has included a presaturation system and multiple solids feed points to allow for more even distribution of the solids across the Filter Bed surface. Dewatered cake concentrations of 35–45 percent have been demonstrated on anaerobically digested biosolids in 5–10 days. A unique four-wheel drive articulated solids retrieval unit quickly removes dried solids from the beds using a rotating belt that minimizes the pickup of sand from the top of layer of the bed. It also levels and aerates the bed for the next application.
A phased construction approach was used by the District to build the Deskins filter beds in 2000 (in operation), and delay the construction of the Class A air drying facilities until 2001. The present worth unit cost (/dry ton) for the recommended facilities was approximately ½ the unit cost of continuing the existing program. The Phase I facilities for the recommended biosolids management alternative Deskins filter beds are anticipated to save approximately 40,000 annual costs in 2001 over the existing solids management program. A 136,500 annual cost savings is anticipated in the year 2002 after the Phase II air drying facilities are constructed.
The air drying process includes the placement of dewatered biosolids on an asphalt pad, shaped into windrows, and periodically turned to dry the solids, reduce volume and produce a soil-like Class A material. The process enhances the natural drying and evaporation of moisture from some dewatered biosolids material by the sun and wind. The City of Louisville and Fort Collins, Colorado and the Pinery Water & Sanitation District are municipalities that have proven that the air drying process can consistently meet the EPA 40 CFR Part 503 Class “A” stabilization and site specific EPA Region VIII permit performance requirements in 2–4 months and has significantly lower costs when compared to other Class “A” stabilization processes, such as traditional aerated static pile composting.
Advantages of the air drying process include: 1) low cost, 2) significant volume reduction, 3) no amendment required, 4) minimum labor requirements, 5) publicly acceptable dry Class A biosolids with a soil-like consistency, 6) maximum flexibility for beneficial use and marketing, and 7) less monitoring and record keeping compared to Class B processes. Air drying is a process that enhances the natural drying and evaporation of moisture from the biosolids by the sun and wind. Numerous citizens, nurseries and landscapers from the Denver Metro area have participated in voluntary pickup programs and have used air-dried biosolids as a soil conditioner for parks, lawns, gardens, golf courses, flower boxes, etc.
Document Type: Research Article
Publication date: January 1, 2001
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