EVALUATION OF A WASTEWATER TREATMENT FACILITY USING A ROTATING LOW-ENERGY NON-BLOWER AERATOR

Transcription

1 EVALUATION OF A WASTEWATER TREATMENT FACILITY USING A ROTATING LOW-ENERGY NON-BLOWER AERATOR Rob Richardson, Ege Richardson, Adrian Hanson, Kurt Moffatt * ABSTRACT The Rincon Wastewater Treatment Facility (WWTP) was designed to serve a maximum population of 775 people and it began its operations in April The current flowrates at the facility are around 10,000 gpd and all the users that are anticipated to be connected to the facility are connected at this time. The facility consists of two rotating aerators (Biowheels TM ) designed to treat an average of 66,000 gpd wastewater. Influent wastewater passes through a manual bar screen as it enters the facility. The influent wastewater then enters the equalization basin, which is continuously mixed using two mixer pumps. The influent pumps operate on a float control and pump wastewater to the aeration basins, where the rotating aerator is located. After the aeration basins, the wastewater enters the settling basins, from which the effluent flows over the troughs to the wet well before being pumped to the land disposal area. The effluent is pumped from the wet well to the leachfield, which consists of chambers (Infiltrators TM ) placed in six different zones that can be isolated using manual valves. This paper presents the performance of this facility since the start-up, including the problems encountered during the start-up as well as the influent and effluent data collected. INTRODUCTION In this paper, a wastewater treatment and dispersal facility in Rincon, New Mexico is described. The treatment facility consists of an equalization tank that also serves as denitrification basin, an aeration tank using Biowheel TM, secondary clarifiers, and an aerobic digester to treat wastewater generated from the community. This paper presents the original design of the Biowheel unit proposed by the manufacturer, the modifications made by the engineer, as well as the treatment efficiency achieved at the plant. The purposes of this paper are: To present the rotating aerator technology, To present the difficulties and process problems experienced during the start-up, To present actual treatment performance of the facility. BACKGROUND INFORMATION Community Profile The Community of Rincon is an unincorporated community in southern New Mexico. The 2000 population of the community is 220. The location of Rincon is indicated in Figure 1. * Rob Richardson, PE, Bohannan Huston Inc., 425 S. Telshor Ave, Ste. 103 C, Las Cruces, NM Ege Richardson, PhD, PE, Aegean Consulting LLC, 5925 San Augustin Dr, Las Cruces, NM Adrian Hanson, PhD, PE, New Mexico State University, Civil Engineering Department Las Cruces, NM Kurt Moffatt, Doña Ana County Utilities Department, Las Cruces, NM 88005

2 Rincon Fig 1. Location Map. Rincon is located above the Hatch/Mesilla Valley floor near the Rincon arroyo s influence with the Rio Grande. The soils in the area are generally well drained loamy soils and fine sandy loams, with moderate to high permeability. The groundwater in the area is 10 to 60 feet below the surface, while it is as shallow as 5 to 10 feet within the valley floor of the Rio Grande River. Due to the development density and highly permeable soils, the long term potential for contamination of the nearby shallow groundwater in the Valley was high. Therefore, the Feasibility Study performed for the area in 2000 concluded to construct a centralized wastewater collection and treatment system to replace the existing septic tanks. The median household income for the Rincon area is around $15,000. The Community s economy is principally agriculturally based. There is little commercial and no industrial activity in the community. Design Basis of the Treatment Plant The Rincon wastewater treatment plant (WWTP) was designed by Bohannan Huston Inc (BHI) and was constructed by Burn Construction in The facility began its operations during April All the users that are anticipated to be connected to the facility are connected at this time. The treatment facility was sized to treat 2020 year wastewater flows originating from the community. Based on the population projections and past census data, the 2020 population was estimated at 776. Doña Ana County, as the grantee and the owner of the system, established a standard per capita wastewater generation rate of 85 gallon per capita per day (gpcd). With consideration for some commercial flows, these numbers translated into a projected design flowrate of 66,000 gpd.

3 Since all the residents and businesses in the Community were on septic tank systems before 2002, no wastewater influent data was available to use as the design basis. Since the Community is predominantly residential with only a few businesses, it was assumed that the influent wastewater characteristics will be similar to the composition of untreated domestic wastewater reported in the literature. The design basis of the treatment facility is included in Table 1. Table 1. Design basis of the Rincon WWTP. Parameter INFLUENT WASTEWATER CHARACTERISTICS Biochemical Oxygen Demand (BOD) Total Suspended Solids (TSS) Total nitrogen Total phosphorus AREA CHARACTERISTICS Annual ambient minimum temperature Annual ambient maximum temperature Annual precipitation Annual pan evaporation Elevation Value 250 mg/l 250 mg/l 40 mg/l 8 mg/l 20 deg F 100 deg F 8 10 inches 94 inches 4060 feet Effluent Permit Requirements Since New Mexico heavily relies on groundwater as the sole source of drinking water, New Mexico Environment Department (NMED) strictly regulates design and construction of leachfields. Based on these regulations, the treated wastewater must either have no more than 10 mg/l of nitrogen in the form of nitrates, or no more than 200 pounds of total nitrogen can be applied to an acre per year. The Facility Plan completed in 2000 evaluated different treatment alternatives that can achieve 10 to 20 mg/l of effluent nitrogen. Cost estimations were performed for these treatment systems with different dispersal areas. Also considering the shallow groundwater in the area, the Plan recommended utilizing a treatment technology that can achieve effluent nitrogen concentrations less than 10 mg/l. Currently, the facility is permitted through the NMED Groundwater Discharge Permit, based on the maximum effluent concentrations summarized in Table 2. Table 2. NMED discharge permit requirements. Parameter Total nitrogen (TKN + nitrate) in the effluent Nitrate, TKN, TDS and chloride in the monitoring wells Daily Maximum 10 mg/l Monitor only (quarterly)

4 DESCRIPTION OF THE TREATMENT PROCESS A Facility Plan was prepared for the community to compare different treatment technologies. The alternatives included rotating aerator Biowheel, facultative ponds, and modifications to the existing septic tanks with an addition of recirculating sand filter. The evaluation considered the capital and operation & maintenance costs of these facilities, as well as reliability and applicability of the system to the community. Considering the County s efforts to establish centralized treatment in the unincorporated portions of the County, and also considering the NMED s concerns regarding the shallow groundwater, a centralized facility which can generate effluent with lower nitrogen concentrations was favored. Therefore, the rotating aerator (Biowheel) technology was selected. Process Flow Diagram The facility consists of two Biowheels designed to treat an average of 66,000 gpd wastewater collected from the community of Rincon. The wastewater collected from the community is transported by gravity to a lift station located north of the treatment plant. The lift station is equipped with a precast manhole and duplex wastewater pumps. Each pump is designed to handle peak hourly flow while the other pump serves as standby. A magnetic flow meter is provided on the discharge side of the lift station to measure the influent to the treatment plant. Influent wastewater from the lift station is transported by a 4-inch pipe to the treatment plant. At the entrance works, the wastewater passes through a manual bar screen with 3/8 inch openings to remove large objects. The wastewater then enters the equalization basin, which is continuously mixed using two mixer pumps. The equalization basin also serves as an anoxic basin to facilitate denitrification and receive nitrified effluent from the clarifiers. The influent pumps in the equalization basin operate on a float control and pump wastewater to the aeration basins, where the Biowheels are located. The Biowheel basins are designed as two parallel trains and can operate one or two basins at a time. After the Biowheel basins, the wastewater enters the settling basins, from which the effluent flows over the troughs to the wet well before final land dispersal. The effluent is pumped from the wet well to the leachfield, which consists of Infiltrators TM placed in six different zones that can be isolated using manual valves. The sludge is aerobically digested, with air supplied using another rotating aerator. Digester supernatant is returned to the equalization basin, and the digested sludge is hauled off-site for further stabilization at a nearby facility before final dispersal. A simplified process diagram is included in Figure 2. Facility photos are shown in Figures 3 and 4.

5 To dosing station and leachfield Influent lift station H e a d w o r k s Equalization / Denitrification Basin Biowheel Basin Biowheel Basin Clarifier Clarifier Digester (RAS / WAS flow Wastewater flow Digested sludge flow Decant flow Digested sludge sent to another wastewater treatment facility for further stabilization Fig. 2. Simplified Process Flow Diagram. Fig. 3. Overall View of the Facility.

6 Fig. 4. Rotating Aerator. Rotating Aerator Technology The rotating aerator technology is marketed as a biological wastewater treatment system which combines the fixed film and suspended culture process kinetics. The most important component of the technology is a rotating wheel that consists of a number of prefabricated pipes with individual specially profiled PVC plates, which when assembled form self-supporting segments. The prefabricated pipes are arranged in a circular manner around a horizontal shaft as shown in Figure 5. The PVC plates provide the surface area for biologically active film as well as an air chamber. The supply of oxygen is provided by rotating the wheel. As pipe wheel segments emerge above the mixed liquor, the liquid inside the pocket drains out and the pipe is filled with air providing necessary oxygen for oxidation of wastewater contents. As pipe wheel segments are rotated into the mixed liquor, the air is compressed and forced into the bottom of the reactor tank. During the downward rotation, a portion of the air escapes and is channeled in the form of coarse or medium fine bubbles to the water. The resulting turbulence, combined with the rotation of the wheel, provides a homogeneous mixing of the liquor in the reactor tank. During the rotation of the pipe wheel, a significant portion of the air is kept within the pipes and provides buoyancy during upward rotation and reduces the energy required to rotate the wheel. The intake of air can be adjusted by the rotation speed of the wheel. Photographs of the aerator are shown in Figure 6.

7 Fig. 5. Pipe Wheel Schematic. Fig. 6. Photographs of the Aerator. Design Summary of the Rincon WWTP The size of the rotating wheels was designed based on wheel surface area loading rate of 2 lb BOD/1000 ft 2 per day, and 1.2 lb NH 3 -N/1000 ft 2 per day. This loading rate can be considered conservative when compared to the surface loading rates of 8 12 lb BOD/1000 ft 2 per day value typically used for fixed film treatment process, such as Rotating Biological Contactors (RBC). The total surface area provided for microorganisms on the aerator was approximately 8,500 ft 2. The wheel diameter is 10.5 feet and contains 24 prefabricated pipes. The rotating pipes are driven with

8 chain by a 2 HP gear motor drive. The wheel can rotate at 0.5 to 2 rpm, providing approximately a maximum of 330 lb of oxygen per day. The aeration tanks are designed based on the food-to-microorganism (F/M) ratio of 0.15 lb BOD/lb Mixed Liquor Suspended Solids (MLSS) per day. Effluent from the aeration basins is transferred by a 6-inch pipe to the clarifiers. One clarifier is provided for each treatment train. Solids withdrawn from the bottom of the clarifiers are either recycled back to the equalization basin as return activated sludge (RAS) or are disposed of as waste activated sludge (WAS). Two RAS pumps that are capable of pumping percent of the influent flow are provided. The WAS is pumped to the aerobic digester, which includes a smaller rotating aerator. The digester is capable of providing 60 days of sludge storage. The digested sludge can be hauled to County septage receiving facility, or can be transported to another County wastewater treatment plant to dry on concrete sludge drying beds before final landfill dispersal. The design summary for the facility is included in Table 3. Table 3. Design summary of the Rincon WWTP Unit ENTRANCE WORKS Influent pumps Manual bar screen EQUALIZATION / DENITRIFICATION BASIN Value Two pumps at 1 hp each 3/8 opening, 24 wide Number One Dimensions 20 x 8 x 10 Equipment Two mixer pumps at 0.5 hp each BIOWHEEL BASINS Number Two Dimensions 12 x 8 x 10 F/M 0.15 lb BOD/lb MLSS-day Biowheel diameter 10.5 ft CLARIFIERS Number Two Surface area 216 ft 2 Surface loading rate 300 gpd/ft 2 RAS pumps Two pumps at 0.5 hp each AEROBIC DIGESTER Number One Dimensions 5 x 8 x 10 WAS pump One pump at 4/10 hp

9 EFFLUENT DISPERSAL The Facility Plan (2000) also evaluated effluent dispersal options for the area. Two main alternatives included in the analysis were subsurface dispersal (leachfield) and surface water discharge. The preliminary cost estimates completed for the two alternative indicated that the surface water dispersal alternative is substantially more expensive than subsurface dispersal due to disinfection requirement. The surface dispersal capital costs were anticipated at around $143,000, whereas that of the subsurface dispersal alternative was around $30,000. Therefore, a leachfield area was constructed for effluent dispersal. A geotechnical investigation was completed for the Rincon WWTP leachfield site to determine suitability of the soil for leachfield. The soils at the site consist mostly of silty fine sand to poorly graded fine sand. The soil is dry in the subsurface until near the water table, which is at 10 to 12 feet. The in-situ soil percolation rate was determined as 1.19 minutes/inch. The total leachfield area provided for the facility is approximately 1-acre. High capacity infiltration chambers were utilized for leachfield construction. The leachfield consists of six zones that can be operated independently. The chambers were placed in a bed configuration with a 12 inches separation between rows. The construction of the infiltration galleries is shown in Figure 7. The effluent from the clarifiers gravity flows into the effluent dosing pump station located at the treatment plant site. The effluent is then pumped into the leachfield based on level control floats. Fig. 7. Infiltration Galleries Constructed as Six Zones. OVERALL FACILITY CAPITAL COSTS The total construction cost of the Rincon WWTP and leachfield area was $509,000 in This is equivalent to $7.70 per gallon for treatment and dispersal of wastewater. This cost is significantly

10 less than package treatment plants that are available in the market for similar flowrates that can provide equivalent treatment to include nitrogen removal. It is noted that this cost is significantly more than the cost of the on-site alternatives considered. However, it was believed that the on-site alternatives could not consistently and reliably produce the required nitrogen removals with effluent total nitrogen concentrations less than 10 mg/l. ADVANTAGES OF THE BIOWHEEL TECHNOLOGY The main advantages of the Biowheel technology can be summarized as follows: Small footprint: The area required for this technology was significantly smaller than facultative ponds, which are amongst the most common centralized treatment systems for small communities. The small footprint was considered an advantage for Rincon where land acquisition was difficult and expensive. Low capital cost: The capital cost comparison completed during the Facility Plan (2000) included three treatment alternatives, namely rotating aerator activated sludge, facultative ponds, and septic tanks with a centralized recirculating sand filter. The study indicated that the rotating aerator technology was the most cost effective technology for the community when effluent nitrogen limitations were considered. Low O&M cost: The Facility Plan (2000) indicated that the rotating aerator technology has higher power requirements compared to facultative pond systems. However, once a mechanical technology is considered in order to achieve better nitrogen removal efficiencies, operating costs of the rotating aerator were estimated to be significantly smaller than other mechanical units utilizing diffused air systems. The difference is mainly due to the low horsepower requirements for the wheel. The power requirements for the rotating aerator system are based on the power required to turn the wheel shaft, as opposed to the power required to push air through a given depth of water, as in the case of blowers. The power requirements of the rotating aerator were similar to that of surface aerators, however, it was expected that the oxygen transfer efficiency of the rotating aerator is better than the typical surface aerators in the market. DESIGN MODIFICATIONS Original layout from the manufacturer included a denitrification area at the bottom of the tank. The manufacturer s design indicated that the reactor basin was provided larger than required and hence, the lower portions of the reactor tank would be utilized for denitrification. The design engineer modified this process by converting the equalization basin to a denitrification tank by adding an internal recycle line from the clarifiers. The second modification to the design included the clarifiers. The standard clarifiers provided by the manufacturer of this technology are not equipped with any scraper mechanism. This practice is mainly accepted to decrease the capital costs. The manufacturer s design included clarifiers with steep bottom slopes and with 9-feet water depth. The maximum depth of the clarifiers was limited with the hydraulic profile of the capacity, and the design modifications included increasing the water depths to 12-feet. The clarifiers were constructed without any scraper mechanisms. It is worth noting that the operator s main complaint on this system is the clarifiers (even with 12-feet depth)

11 and the operational difficulties associated with the clarifiers. The operator indicated that they are too shallow to properly thicken the sludge without scrapers, and they are difficult to keep clean. STATUS AND PERFORMANCE OF THE MECHANICAL EQUIPMENT The mechanical equipment installed at the facility includes the following and their accessories: Influent pumps in the equalization basin BioWheels in the aeration basins and digester Return Activated Sludge (RAS) pumps Effluent flow meter and totalizer Effluent dosing pumps During the start-up period, some changes were made to the mechanical equipment installed at the facility, as described below. Return Activated Sludge (RAS) Pumps Timer During the start-up period, the first modification at the plant included installing a timer for the RAS pumps in order to provide flexibility in their operation. Originally, the operation of the RAS pumps was linked to the influent pumps in the equalization basin. In that configuration, when the influent pumps were pumping wastewater into the aeration basin, the RAS pumps would also pump return sludge to the equalization basin. This configuration limited the capability of the operator to adjust the RAS rates. This problem was aggravated by the clarifiers inability to thicken the sludge in the bottom of the clarifiers adequately. On September 1, 2003, a timer was installed on the RAS pumps, to allow the operation of these pumps independently from the pumps in the influent equalization basin. Currently, the timer settings on the RAS pumps are 30 minutes on and 30 minutes off every day. This configuration also helps avoiding denitrification and floating sludge in the clarifiers. Check Valves for RAS Pumps During the start-up period, the check valves on the RAS pumps were replaced. It was determined that the check valves originally installed at the facility were spring check valves, which are not suitable for wastewater. These spring check valves were replaced with ball check valves on October 14, RAS Pumps During the start-up period, while the problems with the check valves were encountered, the regional representatives for the RAS pumps manufacture indicated that the RAS pumps installed at the plant are not recommended for heavy duty wastewater usage. The manufacturer agreed to change the RAS pumps with a larger model that is a better fit for prolonged wastewater use at no charge. Since the RAS pumps and the influent pumps can pump independently, it was required to ensure that the influent pumps can pump larger flows than the RAS pumps so that the equalization basin is not overloaded. Therefore, the influent pumps originally installed in the equalization basin were moved into the sludge sump as RAS pumps, and new larger pumps were installed as influent pumps. These replacements were performed on October 14, The operators indicated that the RAS pumps and check valves have been working properly without any problems since then.

12 Wastewater Flowrate (gpd) Rotating Aerators During the start-up period, the manufacturer s representative for the rotating aerators (WesTech) contacted the engineer s offices around mid February 2004, and indicated that they would like to evaluate the facility in terms of its aeration performance, and make modifications to the rotating aerator installation. They indicated that based on the construction photos they have obtained, they concluded that the rotating aerator shaft may not have been shipped properly from the factory. They indicated that the shaft angle is not correct relative to the water surface and adjustment of this angle will increase the aeration efficiency of the facility. The manufacturer s representatives scheduled a trip to the facility on May 12, 2004 and made the necessary modifications to the equipment. No modifications were made to the aerator in the aerobic digester. Effluent Flow Meter and Totalizer The plant is equipped with a flowmeter and a totalizer to measure effluent flowrates. Due to the problems encountered with the effluent meter during the start-up, the flow measurements during the first year were based on influent lift station pumping data. The effluent flow meter was calibrated in January 2004 and March The plant operator indicated that the meter flowrates recorded are being evaluated and compared against the influent pumping rates before the data can be considered accurate for plant monitoring and reporting. PLANT PERFORMANCE DATA Flowrate Measurements As described in the previous section, during the start-up period, influent pumping rates and operating hours were compiled by the plant operators to determine the monthly averages. The flowrate data collected since September 2002 through May 2004 is presented in Figure 8. 18,000 16,000 14,000 12,000 10,000 8,000 6,000 4,000 2,000 0 Jul-02 Sep-02 Nov-02 Jan-03 Mar-03 May-03 Jul-03 Sep-03 Nov-03 Jan-04 Mar-04 May-04 Jul-04 Sampling Date Fig. 8. Flowrate Measurements

13 The data presented in Figure 8 indicate that the flowrates observed at the facility has been fairly constant since July The average flowrate currently observed at the facility continues to be around 13,500 gallons per day (gpd). This represents approximately one third of design capacity for each unit rated at 33,000 gpd. Influent Quality Since the discharge permit of the facility only requires effluent measurements, the influent wastewater characteristics are not measured at the plant. Only three influent samples are available at this time. The first sample was collected on 8/7/2003, and BOD, TSS, and TKN concentrations were determined as 210 mg/l, 210 mg/l, and 45 mg N/L, respectively. The second and third samples were collected on 9/24/2003 and 2/11/2004. These two samples were tested for BOD only. The BOD concentrations of these samples were determined as 180 and 207 mg/l, respectively. Based on this limited data, it can be assumed that the influent wastewater characteristics of this facility are typical. Effluent Quality Sampling, monitoring and reporting at the facility are regularly performed as required by the groundwater discharge permit. The facility staff performs sampling for TKN, nitrates, total dissolved solids (TDS), and residual chloride in the treated effluent at least once a month. The effluent data collected at the facility is summarized in Table 4. During the start-up period for approximately a year, the plant has experienced high effluent TKN concentrations, indicating sufficient nitrification was not achieved at the facility. The effluent TKN concentrations observed during this time period were as high as 90 mg N/L, which is unusually high for small municipalities, such as Rincon. This may have been explained by either very high influent TKN concentrations coming to the facility (higher than 90 mg N/L), or by process problems. During this time period, only one influent measurement was completed in August This sample indicated BOD and TSS concentrations of 210 mg/l, and TKN concentration of 45 mg N/L, all of which can be considered typical for municipal wastewater. As such, it can be assumed that the plant was experiencing problems related to growth and health of the bacterial culture in the system. The effluent nitrate and TKN concentrations observed at the facility since plant start-up are plotted in Figure 9. As described in the previous sections, a number of process changes have been made between September-October 2003 to correct the operational problems and manufacturing defects. With the implementation of these changes, along with closer operator attention, the effluent quality improved significantly. The data collected since September 2003 indicates that the plant is in compliance with its discharge permit (see Figure 9). Currently, the effluent TKN concentrations average approximately 3 mg N/L, while effluent nitrates are around 6 mg/l. This resulted in a total effluent nitrogen of 9 mg N/L, which is below the discharge permit limit of 10 mg N/L.

14 Table 4. Effluent wastewater characteristics.* Effluent Concentrations (mg/l or mg N/L) Sampling Date Chloride TSS TDS Nitrate TKN 9/12/ /1/ nd 49 12/4/ /6/ /10/ nd 73 3/10/ /10/ /14/ nd /17/ nd /10/ /7/ /8/ /11/ /17/ /2/ /16/ /5/ /12/ /2/ /7/ /6/ /4/ /5/ /6/ /14/ Average Average since September * Additional data on effluent nitrogen was sampled during dissolved oxygen testing. This data is summarized in Table 5. SLUDGE WASTING RATES AND SLUDGE YIELDS During the start-up of the facility, the sludge wasting was performed based on the MLSS measurements. Currently, the operators waste into the digester from the aeration basin for approximately 10 minutes a day. This practice has been in effect since November 2003.

15 Effluent Concentration (mg/l) Nitrate TKN Jul-02 Sep-02 Nov-02 Jan-03 Mar-03 May-03 Jul-03 Sep-03 Nov-03 Jan-04 Mar-04 May-04 Jul-04 Aug-04 Sampling Date Fig. 8. Effluent TKN and Nitrate Concentrations. The digested sludge from the aerobic digester is regularly transported to another nearby wastewater treatment facility and is dried on the sludge drying beds before landfill disposal. Amount of sludge wasted has been set on a regular schedule since February The operators typically haul 3800 gallons of sludge every other month on a regular basis. The digested sludge usually has TSS concentrations of about 15,000 to 18,000 mg/l, which results in approximately 500 to 600 lbs of sludge every other month. Based on the total amount of digested sludge hauled from the facility, the sludge yield for the last five months (January through May 2004) can be calculated as 0.56 lb TSS per lb BOD applied (based on three influent BOD samples that are available for the facility). While this yield also includes the sludge digestion and solids reduced in the digester, it is higher than the typically accepted values for fixed film systems. Also based on the visual observations that the aerator pipes do not have a significant amount of bacteria growing on them, it is assumed that the rotating aerator process is closer to suspended culture systems than it is to fixed culture. AERATION CAPABILITIES AND DISSOLVED OXYGEN DATA Dissolved oxygen (DO) readings are monitored regularly at the plant. During the first year of the facility, the plant experienced difficulty in achieving sufficient DO levels in the aeration basin. Several DO measurements in the aeration basin indicated values less than 1 mg/l, especially during the hot summer days of New Mexico. It should also be noted that during the plant start-up period, the MLSS concentrations were as high as 4500 mg/l. As a combined result of many different efforts described above, including decreasing the MLSS concentrations, fixing the RAS pumping problems, fixing the aerator geometry, and a closer attention to overall plant operation, the

16 nitrification as well as aeration problems diminished. The main problem was the factory assembly of the aerator. Prior to field modifications, the aerator was rotating at its maximum rotating speed. After the field modification to correct factory specifications, the aerator has been operating at half its maximum speed. Currently, the DO levels in the aeration basin continue to be around 2 to 4 mg/l. The DO measurements collected since March 2004 are summarized in Table 5. The effluent nitrogen concentrations measured during the DO testing are also reported in Table 5. Table 5. DO measurements and effluent nitrogen concentrations DO Concentrations (mg/l) Effluent Concentrations (mg/l) Sampling Date Sampling Time Biowheel Basin Equalization Basin Nitrate TKN 3/5/ /9/ /24/ /30/ /12/ : /12/2004 1: /15/2004 3: /19/2004 9: /22/2004 3: /29/2004 3: /7/2004 4: /14/2004 1: /20/2004 2: /27/2004 3: /7/2004 4: /14/ : /21/ : /28/ : /6/ : /8/ : ENERGY CONSUMPTION One of the main advantages of this plant during the Feasibility Study was the estimated horse power requirements for the facility. The data indicated that the facility should have lower power requirements than an equivalent diffused air system. The energy consumption data for the facility since September 2003 is presented in Table 6. Based on the actual flowrates observed at the facility,

17 the average power consumption for the plant per 1000 gallons of treated wastewater is determined as 14.6 kwh. It should be noted that the plant is currently running one of the two parallel systems. As such, the maximum capacity of this train is 33,000 gpd. Currently, the actual flowrates observed at the facility are approximately 13,500 gpd, which is 43 percent of the maximum hydraulic capacity. Table 6. Energy consumption prior to field modifications of the aerator geometry.* Date Power (kwh) consumption Flow (gallon per month) Power per 1000 gallons (kwh/1000 gal) September , October , November , December , January , February , March , April , May , AVERAGE * The power numbers presented are based on the wheel rotating at its maximum speed. The power consumption of the facility is likely to decrease since the aerator has been rotating half of its maximum speed since the field modification in May CONCLUSIONS After plant start-up problems and field modifications were completed, this treatment plant consistently provided effluent nitrogen values that were less than 10 mg N/L. It is note worthy that one of the two parallel trains has been in operation and this one train has been loaded at approximately 40 percent of its design specifications. One of the major problems of this facility was that the rotating aerator was shipped from the factory with manufacturing defects. Other mechanical problems included swing check valves and RAS pumps. Once the problems were addressed, the technology performed well. Of special note is the aerator performance. The dissolved oxygen levels in the aeration basins have been satisfactory. The energy levels presented in this paper are for the system prior to correction of the aerator geometry problem. During the period reported, the power consumption per 1000 gallons of treated wastewater reported for this facility was the same as that of a Sequencing Batch Rector (SBR) facility of the same age in a neighboring community. It is anticipated that, with the corrections and the reduced aerator rotation, the power usage will drop substantially. The clarifiers for this system add operational complexity, and if possible should be built deeper than recommended by the manufacturer. A deeper basin could provide a larger separation between the

18 sludge blanket and the effluent troughs, and therefore produce better effluent quality with less operational problems. The design of the influent pumps and recirculation pumps should not be coupled in operation. The pump control systems should be kept separate to provide operational flexibility.