Palmer Wastewater Treatment Plant 6.7 Alternative 7: Upgrade Existing Lagoons with New Percolation Bed

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1 6.7 Alternative 7: Upgrade Existing Lagoons with New Percolation Bed Description This alternative considers expanding the capacity of the existing lagoon treatment facility and changing the discharge method to a percolation bed. A new 1,700-foot pressure sewer outfall pipeline would be constructed from the existing wastewater treatment plant to a new underground percolation bed located in the middle of the flood channel. The location of the percolation bed was selected to benefit from the natural erosion protection of a vegetated island. The percolation bed would be a large leachfield similar to a residential septic system except that it would be receiving effluent that has had secondary treatment. An above-ground percolation cell was also explored but deemed not feasible because of freeze-up concerns from the surface discharge. The size of a percolation bed is estimated to be approximately 600 x 1300 (20 acres) and is based on preliminary percolation tests nearby. ADEC has suggested a 2-foot vertical separation from seasonal high ground water. 17 In order to obtain the design infiltration flow rate, the bed piping must be elevated at least 4 feet above anticipated high water elevation. Because of the location of the high water elevation and the need to protect the bed from freezing, the bed would need to be constructed approximately 10 feet high above grade. The bed would be protected from erosion with riprap. The percolation bed would be protected from freezing with insulation. See Figure 31 and 32. A new lift station would be required near the existing outfall terminus to provide positive discharge to the receiving waters. This alternative would likely eliminate the NPDES permit because discharge would now be to groundwater rather than surface water. Groundwater discharges do not require an NPDES permit, but are regulated by the ADEC and EPA. Salmon spawning habitat would no longer be an issue. The exact permit limits are not be known because agencies require a permit application with the design details and calculations before a final limit determination is made. Implementation of this alternative would include the following: 1. Upgrade of the capacity of the pumping and screening equipment in the existing headworks to accommodate 2 MGD. 2. Addition of a fourth aerated lagoon cell to operate in series with the three existing lagoon cells. 3. Additional lagoon aeration capacity. 4. Upgrades to the yard piping at the lagoon to integrate the new fourth lagoon pond into the operation of the existing ponds and to increase the capacity of flows through the lagoon system. 5. Installation of additional UV lamps to provide 2 MGD of disinfection capacity 84

2 6. Construction of a new pump station on the bank of the river to convey treated effluent out to the new disposal location 7. Construction of outfall piping and a subsurface disposal system (as described above) in the river s flood plain. 8. Construction of erosion protection for the existing lagoon site and the disposal field located in the river s flood plain. 9. Installing monitoring wells in the river s flood plain around the disposal field and performing groundwater sampling for contaminates as directed by the ADEC Preliminary Design Criteria The design criteria used for the lagoon treatment option in this report are summarized in Table 16 below. Table 16: Design Parameters for Lagoon Upgrade & Percolation Bed Discharge Parameter Value for 2.0 MGD ADF Lagoon Aeration System Total Air Delivery Summer, cfm 7 12,000 Blower Power Demand, Hp 400 Standard Oxygen Transfer Efficiency, percent per foot of submergence 0.75 Effluent BOD from Pond 4, Winter, mg/l Environmental Impacts A summary of the impacts of this treatment alternative is listed below: 1. Assuming treated effluent percolated into the soil remained below the ground surface, surface waters within the area would not be impacted by the addition of treated effluent. 2. Groundwater within the area would be impacted by the subsurface disposal of treated effluent. Regulatory agencies may impose nitrate limits in area groundwater at a limit identified in a disposal permit. 3. Dewatered waste screenings would be transported and disposed at the Borough Landfill as is the current practice. 4. Biomass and inert solids dredged out of the lagoon ponds would be dried, treated, and applied to land application sites as is the current practice. 85

3 5. Depending upon the River s migration, riverbank protection may be needed in the future to prevent erosion of the lagoon treatment facility and the subsurface disposal field Land Requirements The development of the proposed fourth lagoon pond would occur on the parcel of land to the east of the existing treatment facility. The subsurface disposal field would be located on the island in the River s floodplain. A site development plan with headworks, lagoon ponds, UV disinfection process and disposal field is illustrated in Figure 31. Easements would need to be negotiated, purchased and obtained from the ADNR Cost Estimates Rough order of magnitude, pre-design estimates of construction and operation and maintenance (O&M) costs in year 2007 dollars are summarized in Table 17 later in this report. Costs are based on continued influent pumping at the headworks. Details of the quantities, components, and unit costs used to develop these costs are included in Appendix A of this report Advantages/Disadvantages Advantages: 1. No NPDES permit. 2. The City had operated lagoon treatment for over 20 years and has a good understanding of the process. 3. The state and federal regulations allow higher solids concentration limits for lagoon effluent than for secondary effluent from mechanical treatment processes. 4. Process control for lagoon treatment systems is easier than for mechanical biological treatment systems. Disadvantages: 1. City would have assets located in the 100-year flood plain of the Matanuska River. 2. Percolation cells have a tendency to plug over time. 3. The City could impact nearby water well s with nitrates. 86

4 4. Agencies would likely require a nitrate study and impose nitrate limits at a prescribed boundary around the disposal field with periodic monitoring to confirm compliance. 5. The City s personnel and equipment and/or City contractors may be at risk should emergency maintenance and repairs be required for the treated effluent outfall pipeline or reinforcement of erosion protection of the subsurface disposal field. 6. The City could be exposed to significant claims during construction if the channel location moves or water table significantly changes during construction. 7. When the main channel moves back to the west side of the floodplain (Palmer side) the facility will be stranded. 8. The outfall pipeline construction will be difficult and expensive because the groundwater location is so close to the surface. Because of the granular soils, pipeline trench dewatering will be nearly impossible. Contractors will be placing the pipeline underwater. 9. The other disadvantages are the same as Alternative 5: a. Biological nitrification is retarded by colder temperatures b. The existing lagoons are not lined. Leachate from the sewage and sludge in the lagoons is released from the existing ponds and could eventually impact local wells. 87

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7 6.8 Waste Solids Management All alternatives for wastewater treatment will generate waste solids. Regulations require that disposed sewage solids (screenings and sludge) meet pathogen reduction requirements and/or meet vector reduction requirements. Land applied sewage sludge must also meet certain pollution concentration limits. The following summarizes the types of solids generated, alternatives for ultimate disposal, sludge treatment and dewatering alternatives, and recommendations for a solids management program Screenings and Grit Preliminary treatment will generate waste screenings and grit. The treatment process alternatives considered in this report include screening for removal of larger solid materials that otherwise would degrade the quality of the waste sludge for subsequent beneficial use. The MBR option also requires fine screening to remove solids greater than 1 to 2 mm in size. And, all treatment alternatives except continued use of the lagoon ponds will include grit removal to avoid grit deposition in downstream biological treatment basins. Screenings and grit generated will be dewatered, or washed and dewatered (depending on the equipment used), prior to disposal in the Matanuska Susitna Borough Central (Borough) landfill Biosolids In addition to screenings and grit, biological wastewater treatment generates waste sludge or biosolids. Biosolids is a term that is applied to describe sewage sludge that meets the requirements for recycling or disposal in accordance with the federal regulations for sewage sludge Options for Disposal Options for disposal of biosolids include incineration, beneficial land application, and disposal in a permitted landfill. These are discussed briefly below. Incineration of sludge is comparatively more expensive than the other alternatives, even with the availability of natural gas in Palmer. Sludge generated by most biological treatment processes is over 95% water by weight. There are dewatering processes that are able to reduce the water content of the sludge, however the more common mechanical dewatering processes still produce dewatered sludge that is over 75% water. Incineration would require converting this water to steam and sending it into the atmosphere along with the other products of combustion. This disposal option is usually reserved where no other disposal alternatives are readily available within the vicinity of the treatment plant. Burial of sludge in a landfill is an option only at sites permitted for this activity. The Matanuska Susitna Borough s landfill is operated under permit SWMSB MA expiring November 20, The landfill is currently not 90

8 permitted for receiving sludge. There are no other known landfills permitted to receive sludge operating within the Borough or near to the City at this time. In addition, there are no permitted sludge-only landfills (monofills) known to be operating within the Borough or near the City at this time. In order to co-dispose sewage solids (grit, screenings, or biosolids) with municipal solid waste in a permitted landfill (Municipal Solid Waste Landfill, MSWLF), the following requirements must be met: 1. The sewage solids must be free of hazardous wastes and polychlorinated biphenyls (PCB) in accordance with 40 CFR 261, 18 AAC 62 and 40 CFR The sewage solids must not contain free-liquids as defined by EPA Method 9095 (Paint Filter Test) as described in Test Methods for Evaluating Solid Waste, Physical/Chemical Methods, Third Edition, November 1986 (SW-846). 3. The sewage solids must meet the vector reduction requirement in 40 CFR (b)(11); OR must be treated and stabilized to meet Class A or Class B pathogen reduction requirements in accordance with 40 CFR , AND vector attraction reduction requirements of 40 CFR (b)(1)-(10), as adopted by reference in 18 AAC Biosolids may be disposed by beneficial application to agricultural or municipal lands. Land application of sludge may increase the organic and nutrient content of the soil and enhance its water retention character. To be eligible for land application, the regulations cited above also apply as described further below. 1. The soils to which biosolids shall be applied must have concentrations of metals below, and will remain below, limits established in Table 1 of 40 CFR Biosolids applied to agricultural land or public contact sites must have concentrations of metals lower than the limits established in Table 1 of 40 CFR Biosolids applied to agricultural land or public contact sites must satisfy one of the following conditions: a. The concentration of metals at the application sites shall not exceed the cumulative limits for metals established in Table 2 of 40 CFR , OR b. The maximum concentrations of contaminants in the applied sludge shall not exceed the limits listed in Table 3 of 40 CFR The biosolids cannot be applied to the land designated as endangered species critical habitat, or adversely affect endangered species. 91

9 5. The biosolids cannot be applied to the land during periods when the land is frozen, snow covered or flooded. 6. The biosolids cannot be applied to land within 10 meters of navigable waterways. 7. The biosolids cannot be applied to land in excess of the rate at which the nutrients in the biosolids are used by the vegetative cover for the area. 8. The biosolids must be treated and stabilized to meet Class A or Class B pathogen reduction requirements in accordance with 40 CFR , AND vector attraction reduction requirements of 40 CFR (b)(1)-(10), as adopted by reference in 18 AAC Of the disposal options discussed here (incineration, burial, and beneficial land application), the option that is recommended for Palmer is either disposal at the regional landfill or beneficial land application. Incineration is comparatively more expensive than the other options making it less viable. Burial of sludge is currently not permitted by the Borough at their central landfill. The Borough would have to be solicited and the current permit modified to include disposal of sludge by the 2010 renewal date Sludge Treatment Alternatives Alternatives for stabilizing sludge for beneficial land application or disposal at a landfill include aerobic or anaerobic digestion, heat treatment such as occurs in composting, exposure to elevated ph for sustained periods of time, or a combination of these processes Digestion Digestion is the process whereby microorganisms residing in the sludge are allowed to metabolize the organic substrate in the sludge, and thereby reduce the available substrate to sustain their viability. As a result, pathogen concentrations are diminished and cellular mass is partially converted to byproducts such as carbon dioxide, and water in the presence of dissolved oxygen, and methane and ammonia in the absence of dissolved oxygen. An indicator of this biological stabilization process is the reduction of volatile suspended solids, a surrogate for viable biomass and/or its substrate in the sludge. In smaller communities where digestion is used, aerobic digestion is more common than anaerobic digestion. Anaerobic digestion is somewhat more complex, requiring more equipment to control malodorous gases, digester mixing, and temperature, and can require more operational control than aerobic digestion. Aerobic digestion is process reviewed for purposes of the report. Aerobic digestion uses aeration to both mix and maintain aerobic conditions in the sludge while it is digesting. There are multiple ways to achieve aerobic digestion. If the sludge is 92

10 thickened to a minimum of 4 percent dry solids with a minimum volatile content, the process can be carried out at higher temperatures in small reactor vessels with the heat of metabolism provided by the microorganisms. Other aerobic digestion alternatives include using aerated tanks or ponds as low to moderate temperature reactor vessels, with the lower temperatures requiring longer detention times for stabilization. Tanks are typically insulated and covered. Multiple digester tanks provide the ability to stabilize solids to completion in one vessel while receiving waste sludge in the other. Multiple tanks also allow operations to continue while one is down for cleaning and maintenance. Ponds may be used for sludge digestion. Sludge digestion ponds are typically lined to prevent contamination of area groundwater ph Elevation Elevation of the ph of waste sludge is sometimes practiced where the sludge is land applied to soils with naturally low ph levels. Lime is usually the agent added to the sludge via a pug mill or similar device used to thoroughly mix the lime into the sludge. The ph of the sludge must be maintained above 12 for a period of 2 hours followed by a 22-hour period where the ph of the sludge must not be allowed to fall below This is the method currently used by Palmer Sludge Dewatering Several options for sludge dewatering may be considered for Palmer. They include, freeze-thaw dewatering, gravity sand dewatering beds, gravity belt thickeners, and various forms of mechanical dewatering processes and equipment such as centrifuges, belt filter presses, and screw presses. The dewatering options require polymer addition to the sludge to facilitate release of water from the sludge in processing. Mechanically dewatered sludge solids are typically in the range of 14 to 20 percent dry solids Recommended Sludge Management The recommended sludge management plan that will provide the City with the means of processing and disposing of its waste sludge is beneficial land application or disposal at the landfill. For all upgrade alternatives, except for the lagoon treatment alternative, the recommended sludge management process would include aerobic digestion of thickened waste sludge. Waste sludge from the biological treatment process would be mechanically thickened to approximately 6 percent solids prior to being directed to an aerobic digester. Stabilized sludge can be removed from the digester after a minimum detention period of 20 to 75 days depending upon the operating temperature and solids concentration of the sludge. The digested sludge is then mechanically dewatered with a belt filter press prior to land application. During summer months, mechanically dewatered sludge could be loaded directly into trucks for transport to the land application site. During winter 93

11 months, mechanically dewatered sludge would be stored on the site of the sewage treatment plant under a weather shed roof to keep snow and rain off the stored sludge. Sludge seasonally stored under the shed roof would be removed in the summer months and transported to the land application site for final disposal. For the lagoon upgrade alternatives, the recommended solids management plan would be to continue using the City s suction dredge to periodically remove solids as is the current practice. 94