Poplar Grove Utility District, serving South Tipton County in southwestern Tennessee, built its original one million gallon per day (mgd) water treatment plant in 1981. The district’s former treatment process included aeration, conventional sedimentation and gravity filtration for removing 25 to 30 parts per million (ppm) of carbon dioxide (CO2) and 2.0 ppm of iron from the groundwater supply.
The aeration stage reduced CO2 and imported oxygen into the water for iron oxidation. Reducing the CO2 content raised the water’s pH.
This, in turn, increased the iron oxidation kinetics and helped stabilize the water. The clarification stage allowed for most of the iron to be removed prior to filtration. The clarifier basin was also used to chemically adjust pH for stability throughout the distribution system. Two concrete filter cells provided filtration for polishing the water prior to delivering it to the utility district’s customers.
By 1993, water demand had increased again, as another 1,900 connections were added to the district’s service area. To accommodate the growing demand, an inverted cone clarifier and another filter cell were added to the plant, replacing the conventional sedimentation basins and bringing the total capacity to 2 mgd.
The population served by the district continued to grow throughout the 1990s and, as a result, another plant expansion was soon required to meet demand. When the district’s engineering firm began design work for the expansion, the plant was operating 24 hours a day to meet peak demand periods.
Rather than expand the plant by 1 mgd as in prior construction projects, King Engineering Consultants, Inc. suggested expanding by 2 mgd in this phase to keep ahead of the growth.
Combining new and old
Based on the existing system’s performance, King Engineering and the utility district opted to use the same treatment process of aeration, clarification and filtration processes. The in-place clarification equipment used hydraulic energy for mixing and a cone-shaped tank for effective flocculation and iron settling. Engineers designed more flexibility into the plant so that either the new or existing clarifiers could alternately feed the bank of filters.
The existing gravity filters use a gravel support system with the filtration media on top. Surface washers are used to break up solids collected on top of the filter media and aid in its removal. Though using surface washers helps provide a better cleaning of the filter bed than water-wash only backwash, combined air and water-wash have been found to provide a more thorough cleaning of the full depth of media.
While designing the new filters, the support gravel system was eliminated and replaced with a media retaining shield that is factory installed on the underdrain. The media retaining shield replaces the support gravel layers, saving approximately 11-in of filter depth as well as the supply and installation costs for the gravel. Eliminating the support gravel also alleviated the potential for gravel upsets during combined air and water backwash.
Construction of the expansion project began in August 2001. USFilter General Products, working through its local representative Dickson/Pearson Associates, provided an aluminum-induced draft aerator, cone clarifier and filter parts for two concrete gravity filter cells. By March 2003, the new plant expansion came online.
Although chemical oxidation (using lime) and natural aeration (using a cascade aerator) could oxidize iron, these options would require additional cost over using mechanical aeration. The utility district selected an aluminum-induced draft aerator to provide iron oxidation as well as CO2 removal for the entire 4 mgd plant flow.
With mechanical aeration, raw water CO2 is reduced from 25 to 30 ppm to less than 5 ppm. A gravity distribution tray used to spread the water across the cross-section of the 12-ft square unit minimizes the pumping head required for operation. A countercurrent flow of air is provided by two blowers located on top of the unit. Each blower is constructed with only aluminum and stainless steel components to minimize corrosion. Pre-lubricated and sealed bearings virtually eliminate all maintenance of the blowers.
As the water cascades down through the unit, round PVC slats break the water into small droplets. This allows CO2 to be efficiently transferred out of and oxygen, into the water. The round PVC slats were selected because of their ability to resist fouling by precipitated iron.
As iron builds up inside the unit, the slats can be individually removed for cleaning. After 18 months of operation, the aerator has not yet required cleaning to remove iron deposits.
Chlorine, hydrated lime and an anionic polymer are added to the water after the aerator. Chlorine aids in the iron oxidation process and also provides disinfection. Hydrated lime is added to adjust pH for stability, and anionic polymer builds the iron floc strength and increases settling. The clarification stage follows.
Some plants do not clarify ahead of the filters, but there are significant drawbacks to this. Clarifiers remove most of the heavy solids that would otherwise be placed onto the filter bed. Not using clarifiers would decrease the filter run times, increase the backwash frequency and increase the backwash waste.
Unlike other types of clarifiers with motor-driven moving parts that require maintenance and additional power, Poplar Grove’s inverted, cone-shaped Spiracone clarifier has no moving parts. As water enters the bottom of the clarifier, it spirals upward. The spiral motion in the smaller base volume provides flocculation energy and mixing as water enters the unit.
Measuring only 8-ft, 6-in in diameter at the bottom of the unit to 42-ft at the top, the inverted cone shape provides for a gradually reducing upflow rate as water passes through the unit. Formed solids settle as the velocity decreases. An internal sludge removal hopper captures solids for periodic sludge blowdown and removal from the basin. Clarified water is collected at the top of the unit prior to passing through to the filters.
“The Spriacone clarifier requires very little maintenance,” said Emil Rieben, plant operator of the Poplar Grove Utility District.
The two 16-ft square filter cells added during the expansion incorporated a Multiblock underdrain system that includes one primary and two secondary internal chambers, with compensating orifices located between the chambers for uniform backwash distribution. Final orifices located on the top of the block provide near-uniform distribution of backwash water.
Multiple blocks are connected end-to-end to form a lateral row; multiple lateral rows cover the filter cell floor. A common flume system contains the air wash supply header and connects the lateral rows to the effluent and backwash water supply.
The MS-500 underdrain shield is manufactured by sintering high-density polyethylene beads to form a semi-porous plate. The shield’s porosity retains the filter media but allows water to pass during filtration and air and water to pass during backwash. A 30-in dual-media bed is directly supported on the shield to provide final filtration.
During backwash of the new filter cells, a combined air-scour rate of 3 scfm/sq ft, along with water backwash of 5 gpm/sq ft, are applied while overflowing the two washtroughs in each cell. Combining air and water in such a manner scours adhered particulate from the media grains and flushes them from the filter bed.
During the backwash, media expands throughout the water depth. Low-profile Multiwash baffles are attached to the troughs to prevent media loss during the cleaning process. The baffling system’s geometry allows the troughs to be placed closely to the media surface to reduce backwash waste volume and minimize the depth of filter structure needed to accommodate the troughs.
USFilter provided a media loss prevention guarantee of less than 2-in per year media loss with the filter system. So far, the utility district has been very impressed with the backwash process.
“The concurrent air and water backwash, or the Multiwash process, cleans the filters very well, better than any other system we have seen,” said Rieben.
From 1985 to 2004, the area served by the Poplar Grove Utility District experienced a population surge of 11,000. To keep up with the water demand, the utility district typically upgraded its water treatment plant’s capacity by 1 mgd every decade until, in the late 1990s, the utility district proactively increased its capacity by 2 mgd, bringing the total to 4 mgd. The district currently serves 5,650 connections and 16,150 residents.
Since coming online in 2003, the expanded system has kept CO2 below 5 ppm and iron at or below 0.2 ppm. Both numbers are well below the district’s water quality treatment goals. Whereas the plant once required 24/7 operation during periods of peak demand, it now only runs 16 to 18 hours per day during these times.
“With our recent upgrade, we should be able to accommodate the needs of 8,000 more residents before we have to upgrade the treatment plant again,” said Mike Brewer, assistant manager of the Poplar Grove Utility District.
Brewer recalled that during the 2001 expansion, every piece of equipment was installed with future plant expansions in mind. Ample room was left for two more 1-mgd filters and an additional 2-mgd upflow clarifier.
Because concrete for the filters has already been poured and piping laid for the clarifier, the next expansion is not expected to cost as much as it might have without this careful engineering forethought. Moreover, subsequent expansions are expected to cause fewer interruptions to plant operations.