Filtering in New Technology
Harbor Island Utilities, Inc. (HIU) provides water and sewer service to residences and businesses on the barrier island known as Harbor Island in Beaufort County, S.C.
The water and sewer systems were installed in the early 1980s when Harbor Island was under the control of the Fripp Island Co. Under an agreement between HIU and Fripp Island, the island is obligated to accept and dispose of the HIU effluent. The Fripp Island Public Service District (FIPSD) decided to upgrade its wastewater treatment system to produce “reclaimed water” so it could receive higher land application rates and reduced buffers from the South Carolina Department of Health & Environmental Control (SCDHEC).
The waters surrounding Harbor Island are classified as Outstanding Resource Waters (ORW). This classification prohibits the discharge of any treated wastewater, regardless of quality, although stormwater discharges are allowed. This article describes the technology that HIU decided to use to achieve reclaimed water status.
Reclaimed water is defined in SCDHEC Regulation R.61-9 as having the following characteristics:
- BOD5 £ 5 mg/L;
- TSS £ 5 mg/L monthly average;
- TSS £ 7.5 mg/L weekly average;
- Possible turbidity limits;
- Fecal coliform limits;
- Required TRC residual; and
- Additional limits—case by case.
The regulation allows higher land applications rates with reclaimed water than with normal secondary effluent. In addition, the required buffers can be reduced at the department’s discretion.
The HIU effluent of 0.3 mgd (225 gpm) normally met this quality but could not consistently achieve the tight TSS limitations. HIU evaluated the various filtration technologies and selected the AMF2 Microfiber Filter manufactured by Amiad Corp. The filter is simple, reliable and compact.
At the heart of the system are cassettes about 2.5 x 4 in. Each cassette is wound with multiple layers of high tension polyester threads. Different winding tensions produce cassettes with 2, 3, 7, 10 and 20-micron filtration degree ratings. These cassettes are “plugged” into a stainless steel collection tube to form a 6 ft. long row of cassettes, and there are 35 rows oriented radially around this tube.
During filtration, the filter vessel is full of dirty water. The water seeps through the cassette windings at a very slow rate of 0 –1.3 gpm/ft2 of filter area. This small flux keeps energy losses very low.
Clean water passes out of the cassettes through the clean water ports, then into the collection tube where it is conveyed to the filter outlet flange. The filtration process only requires 3 psi (0.2 bars) to operate with a design maximum pressure of 150 psi (10 bars). When the pressure differential between the inlet and the outlet of the filter vessel reaches 2-3 psi, the PLC will initiate a cleaning cycle. The filter comes offline for 8 to 10 minutes during the cleaning cycle, at which time the filter vessel is drained of all fluid. Each cassette is then thoroughly washed with high-pressure jets of water produced by a booster pump mounted on the filter unit.
As these thin water jets hit a cassette, they pass through the thread windings and impact a splash plate. The back-splash from this plate impact opens and vibrates the threads and flushes debris out of the windings.
In practice, the nozzle assembly shoots a series of jets tightly spaced along the entire length of each cassette. This nozzle then passes down a complete row of cassettes on the cassette assembly, cleaning one side of two rows of cassettes at a time. At the end of the row, a mechanism rotates the assembly 1⁄35 of a turn and the nozzle passes the length of the assembly between the next two rows of cassettes. This continues until the cassette assemblies have made a complete rotation assuring that all cassettes have been cleaned. A slight trajectory shift is made between each cleaning cycle to prevent the water jets from permanently separating the thread windings.
Next, the filter vessel is filled with dirty water. A short purge cycle sends the first filtered water to a drain, flushing out any debris in the lines. The filter then automatically puts itself back on-line.
Three sets of samples were taken using 10µm, 7µm, and 3µm cassettes, and sent to three separate WQ labs for TSS, BOD, and particle size distribution analyses. The analyses below were conducted and reported by the Spectrex Laboratory.
Package WWTP - 0.3 mgd - 225 gpm, secondary sanitary effluent
Two pre-Amiad filter samples were collected and analyzed, one before and one after the pump. As might be expected, there were 5% more particles after the pump than before. Some particles became smaller and some larger. The difference in TSS between the samples was less than 1%, which is an insignificant difference (13.21 ppm pre-pump and 12.22 ppm post-pump).
- 10-Micron Cassette: The 10-micron cassette dropped the TSS value from 12.22 ppm between the pump and filter to 0.11 ppm after the filter, resulting in a TSS reduction of 99.1%. Whereas particles were as large as 77 microns in the post-pump pre-filter stream, the largest particles in the post-filter sample were 10 microns and they accounted for only 0.008 ppm. 2-micron particles were reduced by 52%, 3-micron particles were reduced by 83%, 4-micron particles by 93%, 5-micron particles by 97%, 6 and 7-micron particles each by 96%, 8-micron particles by 99%, 9-micron particles by 100% and 10-micron particles by 99%.
- 7-Micron Cassette: The 7-micron cassette dropped the TSS value from 12.22 ppm between the pump and filter to 0.05 ppm after the filter resulting in a TSS reduction of 99.6%. Whereas particles were as large as 77 microns in the post- pump, pre-filter stream, the largest particle in the post-filter sample was 16 microns. There were a few particles larger than the filtration degree of the cassette being tested. However, this cassette reduced TSS to a greater degree than the 10-micron cassette as expected. 1-micron particles were reduced by 53%, 2-micron particles by 87%, 3-micron particles by 96%, 4-micron particles by 98% and 5-micron particles by 99.5%.
- 3-Micron Cassette: The 3-micron cassette dropped the TSS value from 12.22 ppm between the pump and filter to 0.02 ppm after the filter, resulting in a TSS reduction of 99.8%. Whereas particles were as large as 77 microns in the post- pump pre-filter stream, the largest particle in the post-filter sample was 8 microns. This cassette reduced TSS to a greater degree than the 7-micron cassette as expected. 1-micron particles were reduced by 42%, 2-micron particles by 83% and 3-micron particles by 96.5%.
The total TSS reduction increased with finer filtration degree cassettes as expected. However, for practical purposes the 10-micron cassette, decreasing the TSS from 12.22 to 0.11 ppm (99.1% reduction), is a reasonable choice for assuring that WWTP effluent discharge is meeting all regulatory permits.