Beneficial Use into the Future - A Small Community Perspective
Abstract:The City of Washougal (City) is located in Clark County, Washington, approximately 18 miles east of Vancouver. The City is bordered by the Columbia River to the south, the City of Camas to the west, and the Columbia River Gorge Scenic Area to the east. The 2008 population of the City was approximately 13,480 with a projected growth rate of 4 percent from 2003 to 2012. Washougal's climate is influenced by several major geographic factors. Shielded by the Coast Range to the west and the Cascade Range to the east, all of Clark County enjoys the moderate temperatures of the Oregon and Washington valleys that lie between these two mountain ranges. Annual mean temperature in Washougal is 51° F. Average annual rainfall varies considerably throughout Clark County, ranging from 41 to 114 inches with an annual rainfall for the Washougal area of 50 inches. A high proportion of Washougal's rainfall occurs between October and April, characteristic of the rest of Western Washington, with its mild, wet winters and warm, dry summers.
The City operates a wastewater treatment plant (Plant) constructed in 1998 in its current configuration, and was expanded in 2005 and 2006. The plant utilizes an oxidation ditch for secondary treatment, which is essentially a variation of the activated sludge treatment process. This system operates on a continuous basis, and uses secondary clarifiers with disinfection provided by a UV light system. The plant operates a series of four sludge lagoons that accept waste activated sludge from the oxidation ditch following extended aeration. The lagoons provide solids treatment and biosolids storage.
The City currently land applies Class B biosolids at a property leased from the Port of Camas- Washougal (Port) located within two miles of the plant. Biosolids are land applied on approximately 25 out of 45 acres leased from the Port which is used to grow hay and is harvested during summer months. The site can handle a maximum of 5 dry tons of biosolids per acre per year based on total nitrogen concentrations applied at maximum allowable agronomic rates. The amount of biosolids produced by the City exceeds the capacity of the Port site based on current production of approximately 160 dry tons of biosolids per year. The Port anticipates development of the current beneficial use site and the lease expires on 31 August 2009.
As recently as 2008 the City has evaluated local and distant Class B land application beneficial use options with limited success. In addition, the City evaluated disposing their biosolids into a nearby community Class A treatment facility but due to cost elected not to move forward with this disposal option. Due to growth impacts on available farmland and perception issues raised at public hearings, the City has decided to evaluate and implement long-term Class A biosolids treatment and distribution. Generating Class A biosolids fits well with the City's sustainability efforts and has strong support by their Council. This abstract describes an evaluation that was conducted by the City to study Class A treatment technologies, costs, triple-bottom-line evaluations, distribution, and preferred alternatives for the City.
There were several key objectives of this evaluation, as follows:
Project future biosolids loading in conjunction with recommended improvements to liquid and solids processing at the Plant.
Identify Class A biosolids alternatives that meet the economic, social and environmental goals of the City.
Identify permitting and operational issues, costs, and marketability associated with the proposed alternatives
An extensive evaluation of the Plants flows and loads were calculated using previous planning documents, review of the most current Plant data, conversations with operational staff, and through the use of modeling software. The results of this evaluation indicated that previous projected biosolids production was significantly underestimated. The annual lagoon dredging and land application program at the Port property is not keeping up with the solids production within the Plant. Thus, the solids storage lagoon is quickly meeting its ultimate capacity for solids holding. Twenty-year solids loading is projected at approximately 700 dry tons annually relative to a current rate of 160 dry tons.
The Plant does not currently digest solids wasted from the oxidation ditch. Addition of an aerated digester would more rapidly condition sludge as compared with facultative lagoon treatment to produce a Class A or Class B biosolids product. One of the four lagoons is large enough to be used for aerobic digestion for present loads generated at the plant. However, an aerobic digester in a concrete tank located next to the existing clarifier was evaluated as an option for the City. Either of these options would produce a digested sludge with a better than 2% solids concentration, a 60-day solids retention time (SRT) and better than 38% volatile solids reduction, to achieve a Class B determination or further treatment to reach Class A.
It was recommended digestion be considered part of any long-term biosolids facilities constructed at the City to stabilize the biosolids prior to Class A treatment. In addition, the digestion would provide a Class B backup alternative in the event of equipment failure. Due to the additional expense associated with constructing an anaerobic digester for the relatively low solids loads seen at the Plant, this option was not evaluated and aerobic digestion was selected. The minimum hydraulic retention time for a new digester is recommended to be 60 days. It is possible to construct aerobic digestion with one of multiple options. The two most cost effective options evaluated included:
Convert Lagoon #3 to a permanent aerobic digester by lining the cell and partitioning to reduce excess (unnecessary) volume. This option includes the addition of four mechanical surface aerators, a telescoping valve and pumps for decanting supernatant. Dredging of solids and dewatering would be required to properly install a plastic liner.
Construct a large concrete aerobic digester using un-thickened solids. The volume of the concrete tank would be approximately 525,000 gallons, with a depth of 12 feet. The digester would have external blowers and a coarse bubble diffuser system to distribute the air. This option also includes a telescoping valve for supernatant and pumps to return decanted liquid to the headworks.
The concrete digester alternative would make better use of available space, due to a smaller footprint. However, converting Lagoon #3 had a slightly lower capital cost relative to the concrete aerobic digester. Given the lower operating cost of the aerated digester over time, either option would be similar in cost over a 20-year period.
Dewatering technologies are commonly used in the production of both Class A and Class B biosolids to reduce the solids volume and improve the handling characteristics of the solids to be managed. Currently, waste activated sludge (WAS) exits the oxidation ditch at less than 1% percent solids. Assuming a solids concentration of 0.5% in the WAS from the oxidation ditch, dewatering equipment can increase the solids concentration to between 15 and 25 percent solids, removing between 31.8 MG and 32.3 MG of water annually. Each alternative considered included construction of a new building to house the dewatering equipment and store 3 months of dewatered biosolids adjacent to the dewatering building under a covered area.
This evaluation considered a centrifuge, rotary fan press, and screw press for dewatering. The following alternatives require far minimal operator attention and can be operated unattended for extended periods of time. The belt filter press technology was not evaluated in this report, as it generally produces a lower solids concentration and requires a higher level of operator attention due to the nature of the equipment. The ultimate decision of which dewatering facility should be implemented at the Plant will be dependent on cost, ease of operation, footprint, and historical performance.
Three Class A biosolids treatment alternatives were evaluated for the City. Alternatives were compared based on capital and operating costs, ease of operation, finished product quality, and equipment footprint. Social and environmental considerations were evaluated into a simplified tripe-bottom-line analysis. In practical terms, triple bottom line analysis means expanding the traditional engineering reporting framework to take into account environmental and social performance in addition to financial performance. The three Class A biosolids treatment alternatives evaluated were as follows:
Thermal drying process uses thermal energy to evaporate unbound water from the solids, and has been used to increase solids concentration to above 90 percent. This can greatly reduce the volume for subsequent handling and disposal or beneficial reuse of biosolids. Thermal energy can be applied using heated gas, indirect steam, and air. The dry, friable final Class A product can be sold or given away to a variety of commercial, agricultural, and residential uses.
Aerated Static Pile, Covered Aerated Static Pile, and Containerized Vessel Composting were evaluated as part of this study. Each of these composting processes have advantages and disadvantages ranging from space requirements to capital costs. The treatment process uses the combination of a high nitrogen material (biosolids) and a relatively high carbon source (separated yard debris or hog fuel). The Class A product derived from composting can result in a highly marketable material used for landscaping and other residential and commercial uses.
By using the combination of lime and heat biosolids can be treated to remove volatile solids and potential disease causing organisms. The processes evaluated consisted of adding lime to liquid solids generated from a digester and mixing the slurry to elevate the pH. The liquid slurry is thickened and added to a thermal process utilizing steam heat to kill bacteria and dewatered to generate a Class A biosolids cake product. The Class A product can be sold or given away for landscape, agricultural, and commercial uses.
Based on costs, marketability of the Class A product, consideration of social and environmental elements, and potential for regional expansion the City's preferred alternative is composting with a tendency towards covered aerated static pile technology. This Class A technology had the overall lowest cost and suited the City's desire to have a long-term solution to biosolids management issues.
Document Type: Research Article
Publication date: January 1, 2009
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