Startup and Performance of the World s first Large Scale Primary Dissolved Air Floatation Clarifier Bruce R. Johnson 1 *, Jennifer Phillips 2, Tim Bauer 3, Greg Smith 4, George Smith 5, John Sherlock 6 1 CH2M HILL, Englewood, Colorado 2 CH2M HILL, Phoenix, Arizona 3 CH2M HILL, Corvallis, Oregon 4 CH2M HILL, Tucson, Arizona 5 Evoqua, Waukesha, Wisconsin (formerly Siemens) 6 Pima County Regional Wastewater Reclamation Department *Email: bruce.johnson2@ch2m.com ABSTRACT The Pima County Regional Wastewater Reclamation Department, in Tucson, Arizona, built the new 120 MLD (32 MGD) Agua Nueva Water Reclamation Facility (ANWRF) and started it up in early 2014. This facility includes the first large scale Dissolved Air Floatation (DAF) primary clarifiers in the world. DAF primary clarifiers were chosen for their benefits of low cost, small footprint, improved TSS and BOD removal (as compared to no primary treatment). They also eliminated the need for separate solids thickening, and improved oil and grease removal. However, the risk was that there was no full scale implementations in wastewater that could be used to validate the technology. The DAF primary clarifiers at ANWRF treat both the screened raw sewage and the waste activated solids that are combined at the head of each DAF unit. The DAFs have been found to remove between 50% and 75% of the suspended material, depending on chemical addition, and between 30% and 50% of the influent COD, while reliably producing a thickened solids (primary and WAS) approaching 4 percent. Some removal of soluble organics has also been observed, likely a result of a combination of colloidal coagulation and soluble uptake by the cothickened waste activated sludge. Primary DAF clarification at ANWRF has shown performance similar to conventional primary clarification while combining the function of WAS/Primary sludge thickening and grit removal in a single unit process. Actual operation of the DAF clarifiers has met the goals originally envisioned in the design, but with slightly higher effluent TSS values than hoped for. Continuing optimization at the facility appears to be improving overall TSS removal at lower chemical usage rates. KEYWORDS Primary Treatment, DAF, Dissolved Air Floatation, Cothickening INTRODUCTION Pima County Regional Wastewater Reclamation Department (Tucson, AZ) was facing a mandate from the Arizona Department of Environmental Quality (ADEQ) to reduce the amount of total nitrogen and ammonia in system effluent and to pass the whole effluent toxicity test, as well as increase the treatment capacity to handle the County s growing population. To meet this challenge, the County developed and maintained a Regional Optimization Master Plan (ROMP) for wastewater treatment facilities required through the year 2030. The ROMP Final Report, published in November 2007, recommended the implementation of a number of capital
improvements necessary to achieve regulatory compliance and meet the anticipated capacity requirements for the service area. One of these was the construction of a new facility adjacent to an aging facility that would be decommissioned after the new plant was placed into operation. The County elected to use alternative delivery for the new Agua Nueva Water Reclamation Facility (ANWRF). CH2M HILL was chosen to provide this 120 MLD (32 MGD) facility based upon its design/build/operate (DBO) proposal in November of 2010. The facility saw first wastewater in December of 2013. Figure 1 shows an aerial view of the facility and the effluent requirements are shown in Table 1. Figure 1: Agua Nueva WRF Aerial View Table 1: Aqua Nueva WRF Effluent Criteria Performance Criteria BOD5 TSS Total Nitrogen NH3 Total Phosphorus Maximum Month Design Limit 30 mg/l 30 mg/l 8 mg N/L (5 mo. Geometric Mean) 1.75 mg N/L 1.0 mg P/L E. Coli 4 of 7 days non-detect E. Coli Single Sample Max < 15 / 100 ml ANWRF PROCESS DESIGN AND SELECTION The raw sewage is screened by 13 mm (1/2 inch) bar racks prior to being pumped to a combined aerated grit/flocculation chamber. Effluent from the flocculation chamber goes to the primary dissolved air flotation (DAF) clarifiers, prior to entering the three pass step-feed 5-Stage Bardenpho bioreactor system. Bioreactor effluent goes to secondary clarifiers and then to a disk filter system prior to disinfection with chloramines. The bioreactor step-feed and DO control system is used to keep a target ammonia concentration in the secondary effluent (nominally 1.5 mg/l) for the chloramination system.
Waste solids from the end of the first pass of the bioreactor system are returned to the DAF clarifier feed (ahead of the flocculation tanks) for co-thickening with the primary solids. ANWRF pumps the produced biosolids from the DAF float to the nearby sister plant (Tres Rios WRF, formerly Ina Road WRF) for stabilization and dewatering, thus eliminating the need for a solids stabilization/dewatering system. Figure 2 shows a process flow diagram of the facility. Figure 2: Agua Nueva WRF Process Flow Diagram Selection of Primary DAF Clarification The RFP did not anticipate the inclusion of primary treatment within the proposed treatment train, but neither did it prohibit it. A net present value (NPV) comparison of primary treatment with no primary showed clear benefits. The capital cost was slightly lower with primaries, from the reduced secondary system sizing, and the savings in energy costs as compared to No Primary Treatment were substantial as shown in Table 2. Table 2: Net Present Value Comparison of Primary Treatment 20 Year Energy Savings Capital Cost Total Replacement Costs Final NPV Cost Benefit NPV Primary Clarifier $6.3 $213.1 $206.8 $0.2 $207.0 $7.8 No Primary $0.0 $213.8 $213.8 $1.0 $214.8 Note: Energy costs baseline is the No Primary Option Given this savings it was decided to include primary treatment in our proposal. Consideration was given to conventional primary clarifiers, high rate primary clarifiers, microscreens, and DAF
clarification. A comparison of treatment technologies is presented below in Table 3. This evaluation gave clear indication that DAF clarification had significant advantages for the ANWRF. However, the innovative nature of the concept required CH2M HILL to perform an in depth analysis of the concept. This included bench testing of DAF with various influent additives, and pilot testing at a wastewater treatment plant. Table 3: Comparison of Primary Treatment Technologies PRIMARY ADVANTAGES DISADVANTAGES TREATMENT Primary Clarification Conventional technology Larger footprint and odor control costs High Rate Primary Clarification Small footprint High chemical costs, and changes primary phosphorus removal to chemical based Microscreens Low cost and small footprint Unproven technology, and lower solids removal efficiencies DAF Clarification Low cost, small footprint, improved TSS and BOD removal, eliminates need for solids thickening, improved oil and grease removal No full scale clarification implementation on screened effluent, but numerous installations of WAS thickening of primary solids Figure 3 shows the results of the bench testing, which clearly showed the benefits of supplementing the primary feed with a flocculant. Of interest in that screening was the superior performance of waste activated sludge (WAS), added in proportion to typical primary influent concentrations. The use of WAS as a flocculant provided a further benefit where it would be possible to co-thicken the WAS and primary sludge in a single unit, additionally, the flocculation chamber combined with aerated grit combines the functions of grit removal into the DAF clarification process. Figure 3: Performance of DAF Clarification Bench Testing with Various Additives (based on Raw TSS)
Pilot testing was also done as part of the proof-of-concept and found similar results where it was found that TSS removal ranged between 58% and 85% depending on the influent conditioning with combinations of WAS, ferric chloride, and/or polymer. Full Scale Primary DAF Clarification At Agua Nueva WRF, screened raw sewage is combined with the WAS from biological treatment in a combined aerated grit chamber/flocculation chamber. Ferric chloride and/or polymer can be added if needed. The effluent from each flocculation chamber is directly connected to a rectangular DAF unit. Underflow (primary effluent) is then directed to the bioreactor. Float (cothickened WAS and primary solids) is scraped to a solids storage tank prior to being pumped to the Tres Rios WRF. Heavy solids that settle in the DAF s are raked and periodically flushed to the solids holding tank. The DAF system consists of six 20 foot wide x 60 foot long x 16 foot deep DAF clarifiers. At the average plant flow of 120 MLD (32 MGD), the DAF hydraulic loading rate is 4,444 gpd/ft 2. The clarifiers are fully covered for odor control reasons. At the head of each clarifier is a 20 ft x 20 ft combined flocculation/aerated grit removal tank. Figure 3 is an aerial view of the DAF system with the associated odor control system to the left, and the head of the facility to the right. Figure 3: ANWRF DAF Primary Clarification System ANWRF started receiving wastewater on December 17, 2014. The startup of the DAFs was done in stages as the team learned about this new primary treatment process. There have been five distinct periods of DAF operation. The DAF effluent performance is shown in Figure 4, along with the plant influent flow rates as the plant was brought into service. Figure 5 shows the removals of TSS and COD. Figure 5 shows two TSS removal curves, one is the curve based on the raw sewage TSS concentration, and the other is the TSS removal including an estimate of the TSS added by the WAS return to the DAF influent. The COD removals are based on the raw sewage COD.
Figure 4: ANWRF DAF Effluent Performance Figure 5: ANWRF DAF Removals
1. No WAS, Settling Only. During initial startup, the team was focused on growing biomass in the bioreactor system, so there was no WAS produced, and the actual DAF mechanism was not run. The DAFs were operated as very highly loaded primary clarifiers during this phase, and only settled solids. 2. WAS & Floatation. In the second period, WAS was blended with the primary influent, and the DAF floatation system was started. There were some initial difficulties with the floatation system that were resolved during this period. However, it was found that the WAS only option did not have very high TSS or COD removal efficiencies. 3. Cationic. Polymer addition (cationic) was started during this period to improve the TSS and COD removal. This improved removals significantly, but the overall removal was still not near the target effluent TSS value of approximately 120 mg/l. There was no attempt to optimize polymer addition during this period because it was known that ferric chloride addition would soon be started to achieve the plant effluent phosphorus removal goals. 4. Ferric & Anionic. During this period a combination of ferric chloride and anionic polymer was added to the DAF feed (with the WAS). Ferric chloride addition (15 mg FeCl3/L) was begun to achieve the plant effluent total phosphorus goal of 1 mg P/L. Cationic polymers are not effective with ferric chloride, so the polymer was switched to an anionic polymer (1 mg/l dosage) as well. This switch improved TSS removal to above 65 percent based on the raw sewage and above 75 percent when the WAS contribution is considered. COD removals averaged over 50% during this period. This period also included the acceptance test period of the startup (April 27 th through May 13 th, 2014) 5. Cationics. The effluent TP goal is only a contractual requirement, and not actually required by the effluent permit. It was therefore decided to halt ferric chloride addition, and focus the efforts of the operations team on the optimization of the DAFs without iron addition during this period. The team has been testing different cationic polymers during this period, at various dosages, thus the DAF effluent performance has been variable during this operating period. The need to add chemicals to achieve the DAF treatment performance goals was not expected based on pilot testing at other locations. During the startup of the DAFs, one possible reason for this performance issue came to light. It was felt that the mixing in the flocculation tank may not have been optimum, and that baffling could improve contact between the primary influent and the WAS/polymer. Computational Fluid Dynamic (CFD) modeling was done to check the flow patterns in the flocculation tank as installed, and then retrofitted with an influent baffle system (Figure 6). A significant improvement in the flow patterns can be seen in the right hand illustration with the influent baffles installed. The baffles developed during the CFD modeling effort were recently installed. Preliminary results indicated a large improvement in performance, but there is not yet adequate data to present.
Figure 6: CFD Modeling Results of the Flocculation Tank Without (left) and With (right) Baffles Soluble COD Removal The blending of WAS with the raw sewage under aerobic conditions also reduces the soluble COD (scod) in the primary effluent. It is thought that this is done through two mechanisms. First is the coagulation of colloidal COD. The second is through the actual biological uptake of truly soluble COD by the biomass under aerobic conditions, i.e. storage of truly soluble COD as fat within the biomass. This biological mechanism is well documented in wet weather biological contact and in A-Stage treatment in two stage activated sludge systems. The removal of scod was documented in both the pilot test and seen at the expected levels in the full scale treatment. The design was based on approximately 25 percent soluble COD removal across the primary DAFs. Figure 7 shows the actual scod removal for the entire operating period. The line in Figure 7 is a 14 day running average of the scod removal. It can be seen that it was generally increasing during the period of the test. This is attributed to an increasing amount of WAS being returned to the DAF as the flows to the plant increased. It is not immediately apparent what the balance of colloidal removal and truly soluble removal is across the DAFs.
Figure 7: ANWRF Primary DAF scod Removal Thickening Performance The DAF float is easily able to achieve the design goals of greater than 1.5% solids. The solids from ANWRF are pumped to the Tres Rios WRF approximately 5 miles away, so it is important to keep the solids concentration from going too high, which would overpressure the sludge line. The DAF float actually is produced between 3 and 4 percent solids, which is diluted down to approximately 2 percent by opening the lower heavy solids gates for a set period. This control approach has been very successful at maintaining the sludge concentration within the desired solids range as shown in Figure 8. Figure 8 shows the on-line sludge concentration results during the AT period.
Figure 8: Sludge Concentration Produced by ANWRF CONCLUSIONS The first ever large scale use of DAF clarification technology for primary treatment of wastewater has successfully been started up at the 120 MLD (32 MGD) Agua Nueva WRF in Tucson, Arizona. The primary DAF clarification has shown performance similar to conventional primary clarification while combining the function of WAS/Primary sludge thickening and grit removal in a single unit process. Actual operation of the DAF clarifiers has met the goals originally envisioned in the design, but with slightly higher effluent TSS values than hoped for. Continuing optimization at the facility appears to be improving overall TSS removal at lower chemical usage rates. A number of issues have been addressed during the startup of the DAFs, and there is currently an on-going optimization effort related to flocculation and chemical addition to determine the most beneficial operating parameters.