Performance of a Pilot-Scale Nitrifying Trickling Filter Treating Municipal Aerated Lagoon Effluent
Erik R. Coats1*, Ben Watson2, Kiersten Lee3, Matt Hammer4
ABSTRACT: Colfax, WA, operates an aerated lagoon to achieve which is highly efficient at ammonia removal. Activated sludge compliance with its National Pollutant Discharge Elimination System treatment requires one or more aeration basins in series coupled (NPDES) permit, which currently requires biochemical oxygen demand with primary and secondary clarifiers, mechanical aeration (BOD) and total suspended solids (TSS) removal. However, ammonia systems, and associated recycling/wasting pumps. Moreover, removal may soon be required, and Colfax is considering a nitrifying activated sludge treatment produces excess quantities of sludge trickling filter (NTF) that would allow them to also maintain the lagoons. To obtain data from which to ultimately design a full-scale system, a that requires treatment/management. While activated sludge four-year NTF pilot study was performed. Results demonstrated that an treatment is very efficient and broadly popular for achieving
NTF would be an effective, reliable NH3 removal method and could wastewater NH3 removal, it is not always a practical technology produce effluent NH3 concentrations , 1.0 mg/L. NTF performance was option for smaller communities with limited budgets and characterized by zero- and first-order kinetics; zero-order rates resources. As contrasted with lagoon systems, activated sludge correlated with influent NH3 concentrations and mass load. Utilizing facilities require more specialized operators (and often more data from these investigations it was determined that the pilot NTF operators) and are significantly more expensive to operate and could be reduced by 19%, which demonstrates the value of pilot testing. maintain. Finally, pilot data was evaluated to provide a data set that will be useful to In considering alternative treatment configurations for NH engineers designing full-scale NTFs. Water Environ. Res., 87, 35 (2015). 3 removal, trickling filters (TFs) represent a simpler operation and a potentially less costly technology. TFs, which have been doi:10.2175/106143014X14062131178998 utilized to achieve wastewater treatment for over a century, are non-submerged fixed film biological reactors filled with rock or plastic media over which wastewater is distributed semi- continuously (Tchobanoglous et al., 2014). Historically TFs were Introduction used principally for the removal of BOD from wastewater. Excess ammonia-nitrogen (NH3) in reclaimed water dis- However, TFs can also be used to target NH3 removal, charged to surface water bodies is toxic to certain aquatic specifically treating secondary effluent low in BOD and organisms and can also contribute to accelerated water body suspended solids; TFs used in this application are referred to eutrophication. As a consequence, water resource recovery as nitrifying trickling filters (NTFs), with demonstrated use facilities (WRRFs) are increasingly facing new effluent NH3 beginning in the early 1970s (Parker et al., 1990). Although not a discharge limits. These new permit requirements are particularly new treatment approach, NTFs remain an underused technology challenging to smaller municipalities that have historically at full scale. favored low cost, simple facultative or aerated lagoon treatment The potential benefits to NTFs for NH3-focused wastewater systems focused on the removal of biochemical oxygen demand treatment are many and have been described by others (e.g., (BOD). Lagoon systems are prevalent across the U.S.; in the (Daigger and Boltz, 2011)). Excellent descriptions of biofilm Pacific NW region of the U.S. alone there are 63 permitted kinetics and associated theoretical TF models have been lagoons operating in Washington, 59 in Oregon, and 40 in Idaho developed and presented (e.g. Rittmann and McCarty, 2001); (data obtained from public databases for NPDES-permitted full-scale design guidance builds upon the fundamental mech- WRRFs). Many of these facilities could soon realize NH3 anistics, but employs a more simplified kinetics approach that removal requirements; facultative and aerated lagoons cannot benefits from empirical data (Grady Jr. et al., 2011; Krause, 2010; consistently remove NH3, which will thus require significant and Tchobanoglous et al., 2014). While quality empirical data sets costly WRRF upgrades and expansions. from pilot and/or full scale operations are available (e.g. The most traditional approach to NH3 removal from Andersson et al., 1994; Boller and Gujer, 1986; Goldstein and wastewater has been to employ the activated sludge process, Smith, 2002; Gullicks and Cleasby, 1986; Mofokeng et al., 2009; Parker et al., 1995; Parker et al., 1989; Parker et al., 1997), of the 1* Department of Civil Engineering, University of Idaho, Moscow, ID extensive data sets that were reviewed, only those published by 83844; email: [email protected] Parker and coworkers include detailed kinetic data within the 2 Department of Civil Engineering, University of Idaho, Moscow, ID NTF. The value and importance of such kinetic data within the 83844-1022. depths of an NTF relates to the ability to refine and optimize the 3 MWH, Boise Idaho (at the time of this research was a graduate empirical-based design of full-scale NTFs for site-specific student in the Department of Civil Engineering, University of Idaho). conditions; the NTF design models all integrate and require 4 Water Resource Recovery Facility Manager, City of Colfax, WA. zero- and first-order NH3 removal kinetic parameters.
January 2015 35 Coats et al.
The city of Colfax, WA, located in southeastern Washington accordance with procedures described by Tchobanoglous et al. State, operates an aerated lagoon WRRF to produce reclaimed (2014). Specific design assumptions were as follows: maximum 2 water in compliance with its state-issued NPDES permit, which rate of NH3 removal (rn, max) of 1.5 gN/m d; nitrification half is focused on the removal of BOD and total suspended solids saturation coefficient (Kn) of 1.5 mg/L; coefficient ‘r’ (charac- (TSS). However, a pending water quality assessment of the terizing the decrease in nitrification rate with depth) of 0.2; and receiving water could add NH3 limits to Colfax’s permit (based the transitional NH3 concentration from zero- to first-order on a review of other regional WRRFs, Colfax might realize a nitrification of 4.5 mg N/L. permit limit in the range of 1 to 4 mg NH3/L). Considering the The plastic media selected for the NTF was model CF-1900 excess BOD removal capacity of the existing lagoon system, the manufactured by Brentwood Industries (Reading, PA, USA), city recognized that an NTF would be a potentially significant which provided a specific surface area of 157 m2/m3 (48.0 ft2/ cost-saving alternative to lagoon demolition and an activated ft3). The CF-1900 is manufactured in 0.61m 3 0.61m 3 1.22m sludge WRRF upgrade. However, there are no NTFs currently (2ft 3 2ft 3 4ft) blocks; the pilot NTF plan view cross section was operating in Washington or elsewhere in the region from which established as 1.22m 3 1.22m (4ft 3 4ft). Based on the design to derive valuable performance data to facilitate a potential full- hydraulic and NH3 loading, and following accepted NTF design scale NTF design. The city thus commissioned an extended standards (Tchobanoglous et al., 2014), the required total NTF pilot-scale NTF study at their WRRF site, with the purpose to volume was estimated at 7.1 m3 (250 ft3), yielding a total height collect data and establish results that would confirm for both the of 4.88 m (16 ft). Two NTFs in series, each 2.44m (8 ft) tall were city and the state regulatory authority the ability of an NTF to constructed (with 1.91 cm thick plywood walls), and a total of 16 produce reclaimed water at or below potential future NH3 limits. media modules were required. The resulting surface area-to- Specific objectives of this study were to i) establish NTF height ratio was 0.6 m2/m. The design hydraulic loading rate treatment potential on Colfax’s aerated lagoon effluent, ii) assess (HLR¼1.91 m/h, based on gross plan view area) was at the low NTF capacity over a range of hydraulic and NH3 loading end operating range according to the manufacturer’s guidance. conditions, iii) obtain site- and wastewater-specific NTF kinetic The specific hydraulic loading rate (sHLR; based on total media data within the depths of the operating system, and iv) refine the surface area) was 0.0025 m/h; the sHLR is similar to the design approach applied in sizing the pilot-scale NTF, based on volumetric loading parameter (Rittmann and McCarty, 2001), the study’s data, as related to a future full-size NTF. Not only did but incorporates media surface area. Lagoon 2 effluent is the study provide the city with important design and operational pumped from the settling tube module to NTF1 using a 0.373 data, considering the dearth of NTFs in the region, the kW (0.5 hp) sump pump (Liberty Pumps, Bergen, NY, USA); the Washington State Department of Ecology (DOE; which issues same make/model pump is used to transfer NTF1 effluent to NPDES permits in the state) favorably viewed the project for a NTF2. The pilot NTF utilizes a tipping bucket to distribute potential future full-scale NTF project. This manuscript presents influent over the media, with 50% of the NTF surface area dosed and discusses results from the pilot NTF study. for each ‘‘tip.’’ This dosing mechanism pulsed a relatively large volume of influent wastewater to the NTF, thereby enhancing Materials and Methods excess biofilm sloughing; both Albertson (1995) and Parker et al. Description of the Colfax, WA WRRF. Colfax owns and (1989) raised the concern of excess biofilm accumulation, and operates a wastewater system to receive and treat wastewater suggested a large hydraulic pulse to remedy the problem. The derived from residential, commercial, and industrial services operating dosing rate for the flow rates evaluated in this study located within the city limit. Two aeration lagoons operated in ranged from 6 to38 mm/tip, sufficient to prevent excess biofilm series form the central biological treatment system, targeting the accumulation. A perforated trough system was constructed removal of BOD. Aeration Lagoon 1 has a volume of below the tipping bucket and over the top of the NTF media to approximately 15,140 m3 (4.0 million gallons) and a surface promote uniform hydraulic distribution across the media. To area of approximately 0.4 ha (one acre); it contains 41 ensure that oxygen would not be a limiting nutrient, each NTF submerged tube aerators. Aeration Lagoon 2 has a volume of was fitted with a blower (0.16 m3/min [5.6 ft3/min] capacity) approximately 17,000 m3 (4.5 million gallons) and a surface area mounted at the base of the NTF in the annular space below the of approximately 0.61 ha (1.5 acres); it contains 14 submerged media; regular DO measurements of NTF1 effluent confirmed tube aerators. Air is supplied to both lagoons by three variable- that the residual dissolved oxygen consistently exceeded 5 mg/L speed positive displacement blowers providing 8.5 to 15.6 m3 (the lagoon effluent, which was aerated, exhibited a dissolved (300 to 550 ft3) of air per minute each. At the outlet from oxygen concentration exceeding 1 mg/L). To facilitate kinetic Aeration Lagoon 2 is a 60-degree settling tube module that analysis of the NTFs, three 5 cm (2 in) diameter holes were removes algae before chlorine disinfection. drilled across one side of each NTF tower at heights of 40, 96, NTF Design and Construction. The pilot NTF was designed 162.5, and 208 cm (16, 38, 64, and 82 in) above the bottom of the to receive approximately 5% of the city’s average influent media. wastewater flow; the resulting design flow rate was 2.84 m3/h NTF Operation. The NTF system was operated during the (12.5 gal/min). On an NH3 basis, the pilot system was designed late winter/early spring into summer months over a period of to treat an influent maximum concentration of 25 mg NH3-N/L, four years (Table 1). In year 1 the NTF system was operated at its with a target effluent concentration of 1 mg NH3-N/L. Analysis design hydraulic loading rate. Year 2 operations assessed system of lagoon effluent indicated that the NTF influent total BOD capacity, with the hydraulic loading rate increased by a factor of would be less than 10 mg/L; at this low concentration, the effect 2.7. In years 3 and 4 the hydraulic loading rate was reduced to on NTF performance associated with competitive growth of approximately the original design condition. The operational heterotrophic microorganisms was expected to be minimal period was based on i) anticipated DOE NPDES criteria only (Parker and Richards, 1986). The NTF was designed in requiring ammonia removal during low stream flow summer
36 Water Environment Research, Volume 87, Number 1 Coats et al.
Table 1—Summary of NTF operational criteria.