WWTF Capacity Assessment Project

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Russell Bruce
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1 Wastewater Treatment Facility Evaluation The Richland WWTF was constructed in 1985 to provide primary and secondary treatment for the City's wastewater. Section 3 includes a general description of the plant and its design criteria. The plant has historically met its operating objectives and achieved compliance with the conditions of its discharge permit. In 2003, the City retained Brown and Caldwell to conduct a study to determine the maximum capacity of the plant and its ability to satisfy permit requirements, operational and maintenance preferences, and reserve capacity needs over the ensuing 20 years. This section summarizes the results of the 2003 study. The complete study is included as Appendix C to this General Sewer Plan Update. WWTF Capacity Assessment Project The May 2003 Richland Wastewater Treatment Facility Capacity Assessment Report (Capacity Assessment Report) provided a systematic approach to capacity assessment. Unit process simulations were conducted to represent expected operating conditions. The study was conducted collaboratively with City of Richland staff in recognition of the fact that the City's operations and engineering staff are ultimately responsible for operating the plant and meeting permit requirements. The objectives of the WWTF Capacity Assessment Report included the following: Establish true, overall capacity of the plant with all units in service. Determine the plant's reserve capacity to accommodate growth. Determine potential future operational constraints for each unit process. Develop and prioritize recommendations for plant optimization and upgrade. The project utilized the following approach: Perform wastewater characterization. Develop simulation model to evaluate treatment plant processes. Evaluate of unit process performance under future operating conditions using calibrated model. Wastewater Characteristics The wastewater entering a treatment plant is comprised of a mixture of solid and liquid materials. Some of the material in the wastewater is inert, while some material is organic P:l251461Richland General Sewer Plan Update Page 6-1

2 in nature. The quality of the wastewater determines the amount and type of facilities that are required to treat the wastewater to levels which are safe to discharge. Two of the most common parameters used to describe wastewater quality are biochemical oxygen demand (BOD) and total suspended solids (TSS). BOD represents the amount of oxygen which is required by microorganisms to decompose the solid and liquid organic matter in the wastewater. TSS includes all of the inorganic solid materials suspended in the wastewater that will need to be settled out of solution. Raw wastewater flows and loadings at the Richland WWTF from 1998 to 2003 are summarized in Table 6-1. Table 6 1, Raw Wastewater Flows and Loadings from 1998 to 2003 Parameter Average Annual average flow (AAF)', mgd Peak monlh flow (PMF) '. mgd Peak day flow (PDF) " mgd Peaking factors: PMF/AAF PDF/PMF Annual average BOD load, Ibid 8,792 10,157 9,412 10,381 '10,485 9,681 9,818 Peak monlh BOD load, Ibid 9,316 10,592 10,148 11,813 14,315 12,119 11,384 Peak day BOD load, Ibid 13,335 22,586' 13,686 24,906' 23,856' 19,036 19,568 Peak monlhlannual avg. BOD load Peak daylannual average BOD load Annual average TSS load, Ibid 12,358 14,553 14,128 16,096 13,282 12,350 13,795 Peak month TSS load, Ibid 13,630 17,280 15,299 18,381 18,991 15,514 16,516 Peak day TSS load, Ibid 24,981 34,131 26,831 27,568 46,726 32,042 32,047 Peak monlh/annual avg. TSS load Peak day/annual average TSS load I Measured influenl flow rales include recycle flows. 'Pretrealmenl violations (excessive discharge from local food processor). Average raw wastewater BOD and TSS concentrations and temperatures are given in Table 6-2. There has been minimal change in annual average and peak month raw wastewater flows although peak day flows have declined over this period. The reduction in peak day flows may have resulted from work on controlling infiltration by the City. The flow peaking factors, 1.05 for peak month to annual average flow and 1.27 for peak day to annual average flow, are relatively low and indicate minimal storm flow impacts on the WWTF. Peak month flows typically occur during the summer season, when infiltration due to agricultural irrigation is the highest. As shown in Table 6-1, the current peak month flows are significantly lower than the original design and permitted peak month flows of Page 6-2 P:125146lRichland General Sewer Plan Update

3 BOD. mgll T55, mgil Wastewater Treatment Facility Evaluation Table 6-2. Raw Wastewater Concentrations and Temperatures from 1998 to 2001 Parameter Average Annual average Peak monlh Peak day During peak monlh now Annual average Peak monlh Peak day During peak monlh flow Waslewater Temperature, DC Peak month Minimum month During peak month flow and 11.4 mgd, respectively. Hourly flow rates are not recorded, so that peak hour flow data were not available for comparison. BOD and ISS loadings have increased over the 4-year period. Ihe annual average plant data indicate an 18 percent increase iii BOD loading (or 4.5 percent per year) and a 30 percent increase in ISS loading (or 7.5 percent per year) over the 4-year period. Peak day loadings are notably higher than the annual average loadings, especially in 1999 and 2001 for BOD and in 1999 for TSS. During January 2002, plant staff measured ISS concentrations at manholes upstream of the WWIF. The upstream TSS concentrations were lower than the ISS concentration measured at the plant, which suggested that the influent sampler at the plant was drawing a sample that was over-estimating the magnitude of the influent ISS concentration. In order to check the accuracy of the influent sampler, a second sampler was placed in parallel with the existing sampler to emulate its operation. Also, portable samplers were located at different points to determine the affect on TSS concentration based on sample location. Based on this test, it was observed that there was less variability and lower ISS concentration when samples were collected on the wet side of the WWIF influent wet well, presumably because of the reduced length of sample tubing. In March 2002, the sampler was moved to the wet side of the wet well, and since then, the average influent ISS concentration has fallen, and the range of values has been attenuated significantly. Evaluation of Plant Hydraulics The hydraulic performance of the Richland WWTF was evaluated to identify hydraulic bottlenecks in the plant. This type of analysis is typically carried out from the point of wastewater entry into a plant (the influent pump discharge upstream of the aerated grit P:n.SI46/Richland General Sewer Plan Update Page 6-3

4 chamber) to the discharge (plant outfall in the Columbia River). A steady-state proprietary computer model (PROFILE) was used for the hydraulic analysis. The first hydraulic restriction in the plant was found to occur at 10.2 mgd, when the chlorine contact chamber effluent weir becomes submerged. Submergence of the effluent weir could result in short-circuiting of the secondary effluent through the tanle However, this is not considered a capacity constraint since additional contact time is provided in the outfall pipe. The next hydraulic restriction occurs at 14.4 mgd, when the secondary clarifier effluent weirs become submerged. The current plant flow is much less than this estimated capacity, so the impacts of this hydraulic restriction are not imminent. The subsequent bottleneck was found to occur at 16.6 mgd, when both secondary clarifier influent and effluent structures begin to overflow. Submergence of the primary clarifier effluent weirs was predicted to occur at 18.6 mgd. The restrictions at 14.4 and 18.6 mgd are not considered to be catastrophic events. However, weir submergence may impair clarifier solids removal performance and affect proper flow distribution between the two clarifiers. These hydraulic limitations described above do not pose a near-term problem at the Richland WWTF. If, in the future, the plant were to require more hydraulic capacity, the primary and secondary effluent weirs would need to be modified to pass more flow. This would require raising the effluent weirs as well as the clarifier sidewall. Raising the effluent weirs would increase the upstream water surface elevation even under lower plant flow conditions. The secondary clarifiers influent and effluent structures would also need to be modified by raising the sidewall. The impact of these modifications should be further evaluated, especially during the design of any proposed improvements to the secondary system. Liquid Stream Process Evaluation The liquid stream biological process used by the Richland WWTF is an activated sludge process, consisting of two aeration basins and two secondary clarifiers; both unit processes are operated in parallel and as a single sludge system. The process was evaluated using the activated sludge process simulator, Bio Win, calibrated with data collected during June The findings ofbio Win simulation are summarized below. Primary Clarifier Evaluation The Richland WWTF has two, 85-foot diameter primary clarifiers located immediately downstream of an aerated grit chamber. Screened and degritted wastewater can be split between the two clarifiers. At the design peak day flow rate of 12.3 mgd, the surface overflow rate (SOR) with both clarifiers in service would be approximately 1,085 galltflday. This SOR is considerably lower than the Washington State Department of Ecology Oran~e Book of Sewage Treatment Plant Design guideline peak SOR of 2,000 to 3,000 gal/ft Iday, indicating that there is significant excess hydraulic capacity in the primary clarifiers. The influent flow rate during the week of testing was approximately Page 6-4 P: lRichland General Sewer Plan Update

5 Wastewater Treatment Facility Evaluation 6.3 mgd, resulting in an overflow rate of 1,110 gal/ft2/day for the one clarifier in service. As with all unit processes, a distinction must be made between the hydraulic and process capacities of the primary clarifiers. The hydraulic capacity refers to the amount of flow that can be physically passed through the primary clarifiers. The process capacity is the flow beyond which the ability of the primary clarifier to accomplish its process objective (i.e., removal of settleable solids from the wastewater) is compromised. Testing focused upon removal of settleable solids. The settleable TSS in the primary influent averaged mg/l, while settleable TSS in the primary effluent averaged 11.5 mg/l. The removal efficiency of settleable TSS, therefore, was 90.1 percent [i.e., 1 - ( )], a good level of primary clarifier performance if there is adequate process capacity in the downstream, activated sludge system to oxidize the additional organic load associated with the escaping settleable TSS. Aeration Basin Evaluation In order to satisfy the oxygen demand for BOD oxidation, primary effluent and return activated sludge (RAS) from the secondary clarifiers receive aeration from a mixersparger aerator in one of the two aeration basins before entering the secondary clarifiers. To reduce energy consumption and operating costs, the plant staff operates only one of the two aeration basins during normal conditions. The WWTF Capacity Assessment Report identified constraints in air delivery and mixed liquor settleability in the existing equipment and configuration. The existing turbine aerators lack capacity to provide sufficient aeration of the mixed liquor, and the resulting growth of filamentous organisms reduces the settleability of the mixed liquor further, reducing secondary treatment capacity and contributing to the plant's digester foaming problem. Existing blowers include three 125 hp blowers each rated at 2,800 scfm. However, at about 1,400-1,500 scfrnlaerator, cavitation occurs in the region of the sparger causing the air to "flood" the zone and not break up into small bubbles as desired. This significantly limits the capacity of the aeration basins to accommodate higher BOD loading rates. Simulation shows that there is sufficient blower delivery capacity to satisfy the oxygen demand at higher BOD loadings but the system cannot process this BOD loading because of the constraint in the air delivery system. Turbine sparger-type aerators typically have a standard aeration efficiency (SAE) of about 2.0 to 2.5 Ib oxygenlhp-hr. At an estimated SAE of2.0 Ib oxygenlhp-hr the corresponding maximum standard oxygen transfer rate is limited to approximately 405 Iblhr oxygen per 200-hp aerator, or 810 lblhr per basin. Aeration capacity is sufficient to aerate only about 7.1 mgd with a single basin in service. Secondary Clarifier Evaluation There are two secondary clarifiers at the Richland WWTF, each with a diameter of 13 5 feet and a side water depth of 15 feet. The two clarifiers are both center-fed, with Envirex draft tube sludge withdrawal mechanisms. The historical secondary clarifier hydraulic loading is relatively low. Under average daily flow conditions (6.1 mgd), the SOR with a single clarifier operating is about 440 P:/251461Richland General Sewer Plan Update Page 6-5

gal/ft 2 day, compared with Ecology's Orange Book guideline value of 1,200 gavft 2 day. The peak hour SOR value is lower than the Ecology recommended value.

6 gal/ft 2 day, compared with Ecology's Orange Book guideline value of 1,200 gavft 2 day. The peak hour SOR value is lower than the Ecology recommended value. The clarifier solids loading rate (SLR) limits were determined by a state point analysis using the MLSS concentrations predicted by the simulator for a variety of flow and COD concentration conditions. When using a state point analysis tool, there is no single maximum SLR limit; the analysis simply indicates whether the clarifiers are under- or over-loaded under a defined set of operating conditions. A state point analysis was conducted to evaluate clarifier performance for the cases where one clarifier was in service with one aeration basin, one clarifier was in service with two basins, both clarifiers were in service with one basin, and both clarifiers were in service with both aeration basins. The simulations revealed that with a single secondary clarifier and a single aeration basin operating, the secondary clarifiers will start to become overloaded when flows reach approximately 7.5 mgd. With both clarifiers but only one aeration basin operating, the clarifiers will become overloaded when influent flows reach about 9.5 mgd. With both basins but only one clarifier operating, overload will not occur until flows reach about 11.5 mgd. With both basins and both clarifiers in service, the clarifiers will be able to handle peak day flows without becoming overloaded. Solid Stream Process Evaluation The solids processing train at the Richland WWTF includes a dissolved air flotation thickener, mesophilic anaerobic digesters, and belt filter presses. For mesophilic anaerobic digestion, hydraulic retention time (HRT) is typically the limiting criterion. The Ecology-recommended HRT range is 10 to 20 days. For the 2003 analysis, a minimum digester HRT value of 15 days was assumed. A IS-day HRT is in accord with US EPA's Part 503 regulation for production of Class B biosolids.flows and loadings to the digesters, as well as to other solids treatment processes, were estimated assuming that all thickened waste activated sludge would be sent to the digesters along with the primary sludge. The two digesters can operate in series, with the second digester serving as a sludge storage tank, or in parallel, where limited storage is accommodated within each digester. Each of the two, 60-foot diameter digesters has a volume of million gallons. TSS and temperature profiles collected during digester mixing tests in January 2001 suggested that there was an inert zone occupying up to 9 percent of the digester volume, where the mixing was inadequate and the temperature was below that for viable mesophilic digestion. Therefore, the capacity evaluation used an active digestion volume 9 percent less than the theoretical volume. Digester testing revealed that digester capacity would become constrained when influent flows reach about 9.1 mgd, where the two digesters are operated in series with the second digester serving as a sludge storage tank. Foaming in the anaerobic digesters has been a consistent problem at the Richland WWTF since the plant opened in Brown and Caldwell was asked to investigate the digester foaming problem and recommend solutions. The project team held a foaming Page 6-6 P: lRichland General Sewer Plan Update

7 Wastewater Treatment Facility Evaluation analysis workshop that considered possible causes of foaming, including Nocardia proliferation in the digester feed, toxic substances or persistent surfactants in the influent wastewater, and the digester mixing, feed, and withdrawal configuration and operation. The team conducted a field investigation in which wastewater from various treatment processes was sampled and microscopically examined for Nocardia and related species. The field investigation confirmed that Nocardia was present in high concentrations in the digester foam and in lower concentrations in the digester contents, primary solids, and waste activated sludge, and in the supernatant from the dissolved air floatation thickeners (DAFTs). These findings led to the ultimate conclusion that the foaming results from the growth of Nocardia in the plant's secondary biological processes. Brown and Caldwell's September 200 I Solids Process Engineering Report identified a number of possible digester modifications to control foaming, including modification of the aeration basins by adding an anoxic selector cell to prevent Nocardia growth and inhibit foaming, and four digester modifications to control foaming. The City selected modifying the aeration basin (described at the end of this section) rather than modifying the digesters to address the foaming problem because the aeration basin modifications would remove an aeration basin capacity constraint, increase operational flexibility, and improve the performance of the secondary clarifiers, as well as help control digester foaming. The 2001 Solids Process Engineering Report also noted that Ecology had raised concerns that solids loadings at the Richland WWTF was increasing at a rate such that, within a few years, the plant's solids processing capacity would be exceeded. Brown and Caldwell compared projected loading to these solids treatment processes to the original design loading criteria upon which the plant design had been based. In the case of the DAFTs and the digesters, the projected loads, while high, were found to be well below the original design criteria, and these processes had not begun to approach their design capacity. The original design criteria of the belt presses were exceeded, but that problem was easily rectified by simply operating each belt press for more hours each week. Overall Capacity Assessment In rating treatment plant capacity, Ecology requires that the plant be assessed with all process units (i.e., both primary clarifiers, both aeration basins, and both secondary clarifiers) in operation. Figure 6-1 graphically represents the overall Richland WWTF capacity rating under such conditions. P: /Richland General Sewer Plan Update Page 6-7

8 ~ g ~ ti 260 E ' y"", 2t OI I Blower air supply limited I '. ~,... ~ l>.. \.... I Aerator air supply lim ited (2 basins) I I ' ''.. ~AFT limited '\. '-. ' SLR limited (2 basins + 2 clarifiers) I I Peak hour flow hydraulic limit (sec clarifinf& effstructure) 2~5 2'l0 2t ' 2~ 0 )~., I \.0 Pea"k Month Flow (mgd) I " I"..., '., ~ < Figure 6-1. Estimation of Overall Plant Capacity Rating ' Figure 6-1 indicates that the plant's rated capacity of 1104 mgd will not be reached until approximately However, some of the constraints described above will occur earlier. Removal of these constraints will improve the operability of the plant and minimize operating costs. The corrective measures that will remove these constraints and retrieve capacity are described below. Recommended WWTF Improvements Plant improvements recommended for implementation in the near future include insertion of baffle walls to create separate selector cells at the influent end of one or both aeration basins. A selector will greatly reduce the growth of filamentous organisms, thereby improving mixed liquor settleability, which will increase secondary clarifier capacity. Removal of the existing turbine aerators and installation of a high efficiency fine pore aeration system will increase aeration capacity to permit operation in the cost saving single aeration basin mode at the peak month flow of 1104 mgd. Surface skimming through use of a classifying selector in the aeration basin effluent box will assist positive elimination of nuisance filamentous organisms from the system and minimize the anaerobic digester foaming problem. In the next 12 years, digester mixing and feed/withdrawal improvements are recommended in order to maintain and enlarge digester capacity. Page 6-8 P:l Richland General Sewer Plan Update

Activated Sludge Process Control: Nitrification

Activated Sludge Process Control: Nitrification 60 th Annual KWWOA Conference, Session 1: April 11, 2017 Dan Miklos, Senior Associate, Midwest Region, Hazen and Sawyer Agenda Overview of the Utopia Plant