Wefrbc 2001 manuscript on deskins polymer assisted drying beds and air drying0121b by Jeffrey Widner

WEF/AWWA/CWEA Joint Residuals and Biosolids Management Conference Biosolids 2001: "Building Public Support"

S IMPLE, EFFECTIVE, LOW COS T AND INNOVATIVE BIOS OLIDS DRYING BED DEWATERING/PROCES S ING S YS TEM David W. Oerke, P.E. Senior Project M anager/Associate Rothberg, Tamburini & Winsor, Inc. 1600 Stout Street, Suite 1800 Denver, CO 80202 and Gregory A. Fabisiak Wastewater Systems M anager South Adams County Water and Sanitation District P.O. 597, Commerce City, CO 80037 ABS TRACT 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 M onaco Wastewater Treatment Plant (M WWTP) for the South Adams County Water and Sanitation District (SACWSD), Colorado. This paper focuses on the polymer assisted Deskins AQuick 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 M anagement 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 AQuick 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 AQuick Dry Filter Beds,@ has greatly increased the loading capacity of conventional sand beds by

page 1 of 8

WEF/AWWA/CWEA Joint Residuals and Biosolids Management Conference Biosolids 2001: "Building Public Support"

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 AQuick-Dry@ filter beds allow greater application rates and shorter drying cycle times. The AQuick 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 2 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 AA@ stabilization and site specific EPA Region VIII permit performance requirements in 2-4 months and has significantly lower costs when compared to other Class AA@ 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

page 2 of 8

WEF/AWWA/CWEA Joint Residuals and Biosolids Management Conference Biosolids 2001: "Building Public Support"

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 M etro 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. KEYWORDS Air Drying/Storage, Biosolids Stabilization, ABrown Bear,@ Class AA@, Deskins, dewatering, drying bed, filter bed, PFRP (Process to Further Reduce Pathogens) INTRODUCTION 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 M onaco Wastewater Treatment Plant (M WWTP) for the South Adams County Water and Sanitation District (SACWSD), Colorado. The M WWTP is a trickling filter/RBC secondary WWTP that serves the SACWSD. Originally constructed in 1957, the M WWTP underwent major improvements in 1981. Current (year 2000) average annual and maximum month influent flow are approximately 131 to 158 liters per second (3.0 and 3.6 M GD), respectively. The M WWTP is located near Denver International Airport (DIA). A significant amount of growth connected with DIA and surrounding development is occurring. Future phased expansion should result in an ultimate design capacity of 307 liters per second (7.0 M GD). Future flows in excess of 307 liters per second (7.0 M GD) are expected to be treated at a proposed regional WWTP. The solids handling facilities at the M WWTP include hydrocyclone grit removal, gravity thickening, anaerobic digestion, conventional sand drying beds, and truck transport agricultural land application vehicles. All the biosolids produced at the M WWTP are recycled in an agricultural land application program. Approximately 60% of the total biosolids production is transported via truck and tanker trailer and subsurface injected on farm fields located approximately 56 kilometers (35 miles) from the M WWTP. The remaining 40% is processed on 14 conventional drying beds and surface applied and incorporated on farm fields located approximately 8 kilometers (5 miles) from the M WWTP. The conventional sand drying beds were overloaded and inefficient at dewatering liquid biosolids and supernatant. Typical liquid

page 3 of 8

WEF/AWWA/CWEA Joint Residuals and Biosolids Management Conference Biosolids 2001: "Building Public Support"

biosolids drying time ranged from 2-3 months during the summer to 4-6 months during the winter. Facilities that need to increase the capacity of conventional drying beds will find the polymer assisted filter bed a helpful technology. Smaller plants that find mechanical equipment too expensive and labor intensive should consider this technology. The following are advantages and disadvantages of polymer assisted filter beds compared to conventional sand drying beds. Advantages $ $ $ $ $ $

Reduces labor requirements (when the mechanical solids retrieval equipment is used). Increases capacity without building more beds. Requires one-fourth of the space of conventional sand drying beds. Allows WWTPs to pour and retrieve solids Aall year round@ even in extreme cold or wet weather conditions. Cuts drying time from one to six months to seven to 10 days. M inimizes clogging and ponding.

Disadvantages $ $ $

Somewhat higher capital costs for the polymer mixing equipment and the automated removal system. Additional mechanical equipment requires more maintenance. If polymer is overdosed, the top layer of sand may stick together making it less effective as a filter media.

METHODOLOGY A Biosolids M anagement 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 study also included a comparison of various dewatering methods, such as belt filter presses, centrifuges, conventional drying beds and filter beds. The ten alternatives were screened to a preferred biosolids management alternative using cost and non-monetary selection criteria. The selected alternative included retrofits of the conventional sand drying beds to Deskins AQuick Dry@ sand filter beds, followed by Class A air drying. The Deskins AQuick-Dry@ Technology is a patented dewatering process which modifies existing

page 4 of 8

WEF/AWWA/CWEA Joint Residuals and Biosolids Management Conference Biosolids 2001: "Building Public Support"

drying beds by adding a proprietary flocculator for in-line liquid-solids separation and by providing a proprietary medium that is placed about 15 cm (6 inches) beneath the top of the sand surface. The medium consists of vertical cells that lend structural strength to the filter, provide rapid drainage, decreased drying time, and allow a specially designed biosolids retriever to remove solids without damage to the sand. Solids are normally removed on a five to a 10-day cycle, producing solids cake of 35 to 45%. The operation of the AQuick Dry@ filter bed includes four major steps at the SACWSD plant: 1.

A preparation system that Ainjects@ the polymer into the flocculation device. The polymer accelerates the particle agglomeration and hence increases the total amount of water that can be drained and reduces the amount of water that needs to be evaporated. In-line liquid-solid separation is achieved with a flocculation system (RapidFloc M ixer). This mixer has produced a savings of 40-60% in polymer usage at other WWTPs.

In order to facilitate even application of the solids to the bed, with minimal disturbance of the bed's surface, the Deskins "Quick-Dry" Filter Beds are pre-loaded or saturated. When saturating the bed before the solids are applied, two additional important benefits occur: (A) Saturation allows the solids to flow evenly across the Filter Bed surface. M aximum distribution is achieved in this procedure. A manifold system was installed to facilitate the even distribution. (B) Saturation of the Filter Bed forces the trapped air out of the filter media. When the under drain system is reopened, a natural vacuum, or syphoning effect, facilitates the dewatering of the solids. Decreased drying time occurs, allowing for faster retrieval of the solids. Conventional sand drying beds are not pre-saturated. Regardless of the size and type of substratum filter media, filtrate moves more rapidly through the system (however, size and type of media will cause drain times to vary). Liquids will move more quickly through a saturated system than an unsaturated one. For instance, a sponge when saturated, will allow liquid to pass through it quickly, whereas a dry sponge will not allow water to pass through until it reaches its holding capacity and all or most voids are filled with liquid. An automated valve was located in a wet well that is closed during the pour the cycle and after the pour cycle has been completed the valve should be opened allowing for rapid exiting of water contained in the under drain & substratum. Different layers of aggregates including sand and stone are stratified to produce the maximum efficiency of the filtering process. The stabilization layer consisting of a 5

page 5 of 8

WEF/AWWA/CWEA Joint Residuals and Biosolids Management Conference Biosolids 2001: "Building Public Support"

polyurethane or stainless steel cellular containment system allows for physical stabilization of the media as well as the weight distribution needed when heavy automated solids retrieval equipment. Conventional sand drying beds cannot support such equipment without concrete paving strips. An under drain system is used beneath the gravel layers. This is a lateral network of perforated pipes that is used to collect the filtrate for recycle to the WWTP. The perimeter of the Filter Bed usually consists of concrete walls that retain the solids. The concrete walls are 30 cm (12 inches) in height. A layer of sand placed over a substratum consisting of different grades of gravel. This media allows for 100% bed saturation within 10 minutes from the start of the pour. If the filter bed is pre-loaded with WWTP effluent, saturation occurs immediately. 4.

A specialized, self-contained solids retrieval unit was used. This is a small piece of mobile equipment with wide tracks to avoid compaction to the bed media and typically requires one staff person to operate. The retrieval unit sweeps the entire bed with minimal sand loss. It also levels and aerates the bed for the next pour. The retriever can skim solids at a precise depth controlled by hydraulic levelers. The solids are scraped and augured to the center of the skimmer/loader. A variable speed conveyor transfers the removed material to an attached hopper. After the hopper is full, the unit is driven to a solids storage area where the hopper hydraulically dumps the solids. Larger skimmer/loaders can remove up to 1858 square meters per hour (20,000 sq. ft per hour). Filter Beds can be designed so the solids can be skimmed and transported by conveyor to the side of the bed for storage and additional drying.

Conventional sand drying beds are loaded at a rate of 48-171 kg/square meters per year (10 to 35 pounds of solids/square foot/year). The loading rate for Deskins "Quick-Dry" Filter Beds is significantly higher than sand beds, vacuum assisted, or porous slotted block tiles, without concern for extended time requirements for dewatering. The Deskins Process for SACWSD was designed for 10 kilograms per square meter (2 pounds per square foot x 52 cycles per year or 520 kilograms per square meter (104 pounds per square foot). Pump sizing for solids transfer to the filter beds range from 12.6 to 35 liters per second (200 to 550 GPM ). RES ULTS A phased construction approach was used by the District to build the Deskins filter beds in 2000 with operation starting in October 2000, 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 2 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

page 6 of 8

WEF/AWWA/CWEA Joint Residuals and Biosolids Management Conference Biosolids 2001: "Building Public Support"

$136,500 annual cost savings is anticipated in the year 2002 after the Phase II air drying facilities are constructed. The new Quick Dry Filter Beds have greatly increased the loading capacity at the M WWTP compared to the conventional sand beds by a factor of three or four times. The startup of the filter beds occurred in October 2000 and optimization of the drying bed performance will be completed in November 2000. Therefore, a significant amount of data was not available for inclusion, but will be collected and included at the presentation. Initial results indicate that approximately 90% of the free water exits through the Deskins Filter Beds within the first 12 hours. Solids continue to dewater and the percent of solids expected should be 18 to 22% in 24 hours and 45% and higher in 5-10 days. The final solids concentration has varied based upon the climatic conditions and the time the solids remain in the bed after most of the water has drained through the sand. The initial polymer consumption has been high and ranges from 15-25 pounds per ton. DIS CUS S ION Design criteria, use and performance of "Quick-Dry" Filter Beds are affected by several different factors; solids type, conditioning, solids application rates and depths, dewatered solids removal techniques, and climatic factors. The type of solids applied and the effectiveness of chemical conditioners will play a part in the degree of dewatering of the solids and the operation of the filter beds. The solids must be adequately conditioned in order to flocculate the solids and release the free water, since the majority of the dewatering is through the filter media. The Deskins process utilizes an in-line polymer preparation system that injects the polymer into a special flocculation device. Typical problems of standard sand drying beds include chemical overdosing, sand blinding with unattached polymer, and large floc particulate matter that can settle too rapidly and also blind the media. The Deskins flocculation system utilizes equipment that provides more complete and precise polymer contact with the solids helping to eliminate many of these problems. CONCLUS IONS The Deskins filter beds have been demonstrated to be successful for simple and low cost biosolids drying bed system. Dewatered solids concentrations of 45 percent have been achieved in five to 10 days. The loading capacity at the M WWTP compared to the conventional sand beds has increased by a factor of three or four times. SACWSD is saving approximately $40,000 of annual costs over the previous liquid biosolids transport and land application program.

page 7 of 8

WEF/AWWA/CWEA Joint Residuals and Biosolids Management Conference Biosolids 2001: "Building Public Support"

The second portion (air drying facilities) of the biosolids dewatering/processing system is to be constructed in 2001. It is anticipated that the Deskins filter bed will dry the solids to 35-35% initially and will be further stabilized and dried to 70-80 percent solids using the air drying pad in approximately a 1-3 month period. This product will meet that site specific EPA Region VIII AClass A@ stability requirements given favorable weather conditions. The Deskins filter beds and the combined drying bed dewatering/air drying processing system should be considered by other small to medium-size WWTP's with existing conventional sand beds. The processes work best in semi-arid weather conditions similar to Colorado (<38 cm or 15 inches rain per year) but has been shown to be effective in wetter climates in Chicago, Illinois and M iami, Florida. ACKNOWLEDGMENTS The authors would like to thank the following people who contributed to the success of this project and the compilation of information for the paper: 1) M r. Larry Ford (District M anager), Don Ramig (retired assistant manager), and South Adams County Water and Sanitation District Board members for their support of this project; 2) Ernie Travis, Dave Pavlacky, Joan Chavez and Blair Corning, Williams-M onaco WWTP staff, for compilation of current and additional data on the dewatering process; 3) John Rickermann, Rothberg, Tamburini and Winsor, project engineer for Biosolids M anagement Study and Derald M eineke, design project engineer, now working for Black & Veatch; 4) M s. Diane Garvey, Garvey Resources, Inc. for her contribution to the paper; and 5) Dave Deskins, F. D. Deskins, Incorporated for providing information on the filter bed technology.

page 8 of 8