Plant Spotlight: City of Asheboro WWTP

This Plant Spotlight was originally printed in the Spring 2016 issue of NC Currents magazine.

General

Located in the ‘Heart of North Carolina’, the City of Asheboro is the largest city in Randolph County.  Its beginnings date back to the 1780’s and it now has a population of approximately 25,000. In addition to its industries, Asheboro has the distinction of being home to one of the world's largest natural habitat zoos with over one thousand animals.

The City of Asheboro currently operates one WWTP that is located north of the city. It was originally put into operation in 1962 and underwent expansions in 1975, 1986 and 1996. It currently employs 21 O&M, 5 Laboratory & 4 administrative personnel. The plant has an annual operating cost of approximately $2.4 million.

The plant treats both residential and industrial wastewater from the city and the surrounding community.

Since 2005 many of the major industrial users have shut down or moved. During this time, the plant influent went from 70% industrial to 70% domestic. This decline of industry resulted in a loss of 3 MGD in daily average flow. There are currently 13 significant industrial users.

The plant’s effluent discharges into Hasketts Creek and has a design flow of 9 MGD. Currently the plant operates with an average daily flow of 3.0 MGD. During significant rainfall events, the plant has experienced a peak flow of 24 MGD.

Treatment Processes

The wastewater effluent is permitted for the following limits:

• BOD limit 5mg/l : 10mg/l
• NH3 limit 2mg/l : 4mg/l
• Permitted to “monitor only” for total N and Phosphorus

The basic treatment processes include:

Preliminary Treatment

Wastewater enters the Asheboro Treatment Plant through two main collection lines.  The 36-inch sewer line flows into the plant by gravity and the 24-inch gravity sewer is pumped through the spiral lift station into the plant.  The two flows combine at Junction Box #1.  

Influent from Junction Box #1 sewer system enters the preliminary treatment structure through a 42-inch ductile iron pipe.  The wastewater passes through an automatically cleaned bar screen then a 15 MG Vortex Pitas Grit System.  Both the screenings and grit are then removed by a single belt conveyor.  Once this occurs, the wastewater flows by gravity through a parshall flume (20 MGD rated) where the influent flow is measured by an ultrasonic flow meter.

The wastewater from the preliminary treatment then flows into the primary splitter box, where the flow is split, going to a 3 MGD rated Circular Primary Clarifier, and six Rectangular Primary Clarifiers rated at 1 MGD each. The settled wastewater then flows into the trickling filter recirculation pump station.

Primary Treatment

Settled primary sludge is collected by scrapper collection equipment into a sludge hopper located at the inlet end of the rectangular tanks and the center of the circular tank.

Primary sludge is pumped out of the circular tank automatically set by timers, and removed from the rectangular tanks automatically by air, which is controlled by a PLC.

Scum is removed from the surface of all tanks, removal pipe on rectangular tank manually, and by scrapper into a hopper on the circular tank.

Secondary Treatment

Secondary biological treatment consists of a recirculation pump station, trickling filters, recirculation junction box and secondary clarifiers.  The primary settling tank effluent flows into the recirculation pump station.  The recirculation pump station consists of four centrifugal pumps rated at 6 MGD.  These pumps are utilized to pump the primary clarifier effluent and the recirculated trickling filter effluent to the trickling filters.

There are three (3) trickling filters, which are operated in parallel.  Rotating distributor arms are used to distribute the flow evenly over the surface of the filters.  As the flow trickles through the stone media, the biological growth on the stone surface absorbs the organic material in the wastewater, thus reducing the BOD in the wastewater.  The filter drainage is collected in an under drain and flows to the recirculation junction box and filter recirculation valve vault.  The recirculated flow goes to the recirculation pumping station while the remainder of the flow discharges into the secondary clarifiers.

There are two trickling filter recirculation patterns, one with trickling filter effluent back to filter influent with primary effluent and one with secondary effluent back to filter influent with primary effluent.

There are four secondary clarifiers, which are utilized to settle the trickling filter humus.  In general, 0.3 to 0.5 pounds of humus are produced for every pound of BOD removal in the trickling filter.  The sludge is removed by air on set timers.  Scum is removed to the scum pipe by the returning collector flights, and then rotating the scum pipe.  Total volume of all four tanks is 108,066 cubic feet.  The total linear footage of the weir is 816.

Tertiary Nitrification Treatment

Secondary clarifier effluent flows to two single-stage nitrification activated sludge trains.  The nitrification trains consist of a circular aeration tank and a circular center feed, peripheral take-off final clarifier.  Return sludge from the final clarifiers’ mixes with the secondary clarifier effluent in the wet well of the nitrification pump station and is pumped to the nitrification splitter box through a 30-inch DIP.  The splitter evenly divides flow into the two nitrification basins with subsequent flow to the clarifier splitter box through a 36-inch DIP.

The nitrification pump station consists of four variable speed pumps with 7 MGD capacity.  

Aeration and mixing in the nitrification basins are accomplished by the Schreiber process, whereby air is introduced at the bottom of the aeration tank by a series of diffusers, which are suspended from a rotating arm.  “Counter-current aeration” is achieved by providing the air through the rotating diffusers.  In this manner, the manufacturer claims that the air is in contact with the wastewater longer and, consequently, more efficient oxygen transfer is achieved.

The Asheboro activated sludge system is operated under conditions to foster growth of specific types of bacteria, which will convert ammonia-nitrogen (NH4-N) to nitrate-nitrogen (N03-N).  Nitrification is needed at the WTF because the conversion of NH4-N to N03-N requires a lot of oxygen, which would be taken from Haskett’s Creek if bacteria in the creek were to perform the nitrification.

The two nitrification aeration basins have a capacity of 2 MG each, with air provided by three variable speed centrifugal blowers, one up to 1,200 cfm, and two up to 5,000 cfm each, and six positive displacement blowers 760 cfm each.  Mixed liquor exits each aeration basin through a mixing chamber.  The mixing chamber is provided to allow chemical addition, if required.

Calcium Hydroxide may be added to the nitrification splitter box, if needed, due to alkalinity and pH depletion by nitrification reactions.  

The mixed liquor then flows to the clarifier splitter box.  The clarifier splitter box splits the flow to the three clarifiers.  Polymer may be added to the discharge of the clarifier splitter box to assist in precipitation in the clarifier, if needed.  The discharge from the splitter box then flows to the center inlet well of the clarifiers.  Settled sludge is continuously removed from the clarifier bottom through hydraulic suction-type collectors to the center sludge well.  From there the sludge flows by gravity to the return sludge valve vault.  The sludge then can be recirculated to the nitrification pump station through the return sludge valve vault.  Sludge is wasted through a 6-inch gravity flow line from the return sludge valve vault to the sludge conditioning building.  The waste sludge is metered utilizing an ultrasonic flow meter.  An ultrasonic flow meter is installed in each return sludge line within the return sludge valve vault.  Return sludge flow rates are adjusted by hydraulically activated plug valves within the return sludge valve vault. Skimmers remove floating solids from the surface of the water in the clarifiers.  Skimmings flow by gravity from a common sump for all clarifiers to the secondary clarifiers.  The three final clarifiers have a total volume of 211,000 cubic feet with a total of 754 weir linear feet.  

Tertiary Sand Filters

The final clarifier effluent then flows by gravity to the tertiary sand filters.  The Parkson Dynasand Filter provides a unique filtration system since it continuously cleans the sand bed while filtering liquid suspensions to the designed effluent quality.  Feed water is passed upward through the sand bed, exiting from the top of the filters as clean water.  At the same time, sand is continuously removed from the bottom, cleaned and returned to the top.  A small portion of the filtered water is used to wash the sand and leaves the filter as reject water flowing into a wet well where it is pumped to the primary splitter box.  The feed is introduced into the filter through the feed pipe.  The feed then passes down through the feed pipe to the distribution radials where it is introduced into the sand bed.  The distribution radials serve to distribute the feed water flow evenly across the sand bed.  From there, feed water passes upwards through the sand bed, being cleaned in the process.  The clean water (filtrate) passes over the filtrate weir at the top and leaves the unit.  The unit consists of four units of eight cells each with a maximum filtration rate of 8 gallons per minute/square foot. The effluent from the sand filters flows by gravity to the chlorine contact chamber where disinfection takes place.

Innovative BNR Process

In 2011, the plant personnel began exploring methods to perform BNR with the existing equipment.  They found that the process didn’t have enough BOD to denitrify. To address this issue, it was decided to form a partnership with one of the local industries, MOM brand Cereals. In the cereal manufacturing process, high strength sugar water is produced. So in order to improve the BNR process, the WTF now trucks in this sugar water and feeds this carbon source during the denitrification process.  This enables the plant to nitrify and denitrify in the same tank by cycling air on and off every 2 hrs.  Total N then dropped from a monthly average of 20 mg/l to 5 mg/l, with the lowest being 1.87 mg/l.  Total P then dropped from a monthly average of 1.0 mg/l to .07 mg/l.
To accomplish this process modification, the plant purchased a 12,000 gallon tank, 2 tanker trailers, feed pump, coriolis flow meter, nitrate sensor, and an ammonium sensor. In addition to this, some minor modifications were made to the existing SCADA system.

Solids Process & Handling

The solids treatment process consists of a Dissolved Air Flotation (DAF) system to thicken the sludge.

Once it is thickened, the sludge is pumped to anaerobic digesters (Thermophylic). After digestion, it is then dewatered utilizing two-1.5 meter belt presses. The filtrate from the presses can exceed 400 mg/l NH3.

The biosolids management program consists of land application using a private contractor.

SCADA

The plant automation system consists of a central SCADA system that controls and monitors the process equipment, instrumentation, valve actuators, motors, pump VFD’s, etc. The aeration system is controlled by blowers that receive an input signal from the dissolved oxygen probes.

Disinfection

During normal operating procedures, Sodium Hypochlorite (chlorination) is fed to the weirs of the three final clarifiers for algae control and added contact time.  It is also fed to the trickling filter influent for control of filter flies (Phychoda), the sand filter influent and then finally to the chlorine contact chamber. In order to meet EPA discharge limits dechlorination is required. This is accomplished when Sodium Biosulfite is fed at the end of the chlorine contact chamber. Once the effluent leaves the contact chamber it flows to the Cascade Aeration system where it is reaerated before discharging into Haskett’s Creek. The chlorine safety record at the plant has been excellent.

Laboratory

The Water Quality Department operates a certified laboratory at the Wastewater Treatment Plant. The laboratory is administered by a Water Quality Manager, Assistant Water Quality Manager, Chemist, Biologist and a Laboratory Technician. All employees are cross trained to perform multiple physical, chemical and biological analyses of water and wastewater parameters for plant operations, industrial survey and state and federal authorities. All analysts hold a degree in biology, chemistry or environmental sciences.  They are certified in "Bacteriological Methods in the Analysis of Drinking Water", "Process Control Chemistry" and as "Class IV through Class II Wastewater Laboratory Analyst."

The Water Quality Department is certified to perform all testing parameters for the NPDES Wastewater Treatment Plant permit, as well as, Monthly Operating Report for the Water Treatment Plant, except for a few quarterly special testing parameters. The Water Quality Department collects and analyzes more than 67,000 analyses per year.

Awards

• 2010 AWWA NC Central Region O&M award winner
• 2010 (John Stake) & 2013 (Chris Schadt) NC Operator of the year award winners
• 'Treatment Plant Operator’ (TPO) magazine article in the November 2011 issue

Future Plant Goals

• Lift Station Flow Meters for I&I
• More accurate control of Aeration Basins
• Thermophylic Digestion
• Class “A” biosolids
• Feed grease, fats, and oils and excess sugar water directly to digester. 
• Increase methane production
• Use methane to fuel generator, which will supply power for entire plant.
• Zero carbon footprint
• Optimize the plant further with additional sensors and instrumentation for monitoring and control
• Plant permit renewal in 2016

Aeration building.

Dissolved air flotation system

Digester

Headworks

Lab

SCADA