Large Anthracite Media Does the Job
The studies evaluated the total suspended solids (TSS) performance and biological oxidation demand (BOD) of different media sizes and arrangements. The full scale operating systems benefit from the higher solids holding capacity, lower average value headloss and lower percentage of backwash recycle using large anthracite media.
In 1994 U.S. FILTER/General Filter performed a series of pilot tests for Henrico County, Virginia WWTP and Black and Veatch. The overall objective of the pilot study was to select a filter media for installation in a new declining rate granular filter.
An automated GF MULTIWASH filter pilot unit was selected for both site tests. The pilot plant consists of a ten inch filter cell with a MULTIWASH system, effluent and backwash water pumps, airwash blower, two turbidity meters, backwash storage tanks and a PLC driven control panel.
The specific objectives were to demonstrate TSS and filter run lengths using 48 inches of single 1.4 to 1.6 mm effective size (E.S.) anthracite media and 48 inches of single 0.8 to 1.0 mm E.S. sand. A declining rate system was selected to take full advantage of the existing low hydraulic profile. The pilot tests were conducted at 2.5, 3.3, and 5.7 gpm/ft2.
Overall TSS removal percentages were similar for both sand and anthracite, but filter run lengths were shorter using the sand. The average solids capture ratio for the anthracite was 0.19 lb. TSS per square foot per foot headloss, which was double that of the sand. The use of simultaneous and sustained air/water backwash was also demonstrated to provide effective cleaning of the filter media and acceptable filter run lengths.
The anthracite media was then piloted with coprecipitation to compare and evaluate the filtration performance. Coprecipitation using Alum produced the longest filter run and a higher solids capture ratio compared to all other piloted media schemes. The Alum dosage of 3 ppm achieved and average of 75% removal of TSS.
The Henrico County pilot test data demonstrates the MULTIWASH process using large anthracite achieves average effluent TSS concentration of 6 mg/l or less when challenged with an average influent TSS concentration of 17 mg/l or less. It was concluded that the addition of Alum would improve the TSS removal.
The full scale U.S. FILTER tertiary treatment system installed consists of four four-cell General Filter MULTIWASH CenTROL gravity filters with 48 inches of 1.4 to 1.6 mm anthracite media as recommended by the study. The hydraulic loading rate through the filter is 5.0 gpm/ft2 based on a peak flow rate of 96.5 MGD. The system utilizes a backwash water rate of 12 gpm/ft2 and an air rate of 4 cfm/ft2. The waste plant blends its secondary effluent with TSS ranging between 6 to 13 mg/l. The operating system is producing an average effluent TSS of close to 4 mg/l as predicted by the pilot study.
A second study conducted for Harrisonburg-Rockingham Regional Sewer Authority and for Dewberry & Davis Engineers at the North River Wastewater Treatment Plant in Mt. Crawford, VA showed similar results. The pilot study was conducted to evaluate media type, filtration rate, and determine if varying TSS and BOD loading rates at peak flow would meet the plants effluent requirements.
Predicting optimum performance of a full scale advanced water treatment tertiary gravity filter was the main objective. A key consideration was the need to produce a very low level of effluent BOD less than 5 to 6 mg/l monthly average. For a typical secondary biological effluent BOD/TSS ratio of 0.60, the TSS in the effluent must be maintained below 3.3 to 6.7 mg/l.
Testing was conducted on 30 inches of 1.4 to 1.6 mm E.S. anthracite media and 36 inches of dual media using 12 inches of 0.45 to 0.55 mm E.S. sand and 18 inches of 1.4 to 1.6 mm anthracite. TSS, BOD terminal headloss and filter run lengths were evaluated. During the study the normal influent TSS levels of 8 to 12 mg/l produced excellent effluent BODs of 2.6 to 4.8 mg/l. When influent levels reached 21 mg/l TSS effluent BOD was 6.5 mg/l. This indicates that direct filtration will produce effluent BODs below 5 mg/l, if the influent TSS are maintained at about 15 mg/l or less. The average effluent BOD was 4.1 mg/l, meeting the goals of the study.
Based on the results of the pilot tests, the following conclusions were made:
- A conventional, downflow gravity filter containing 30 inches of 1.6 mm E.S. anthracite, operating at an average filtration rate of 4.3 gpm/ft2 or less with influent TSS of no more than 15 to 20 mg/l was demonstrated to be fully capable of:
- Producing very low levels of effluent TSS in the range of 2 to 4 mg/l, which correlates to effluent BOD's less than 5 mg/l on a monthly basis.
- Producing less than about 4% backwash recycle at a terminal headloss of 5.25 feet of water.
- Placing a 12 inch layer of 0.5 mm sand below the coarse coal might produce slightly lower effluent TSS levels, but backwash recycle would be significantly increased.
Actual Performance of the full scale tertiary filters are very good. The plant operates at less than 4 p; 6 mg/l BOD. Peak design flows are yet to be reached with less than half of the peak flow actually running through the plant (approximately 2 gpm/ft2 loading rate). Filter cells are backwashed on a "timed" basis once per day as opposed to initiating backwash based on terminal headloss. This 24 hour run length still equates to about 4% backwash recycle rate which could be cut in half if the filters were run for 48 hours between backwashes.
Both pilot studies effectively demonstrate the advantage of large E.S. anthracite media for the reduction of TSS and BOD in wastewater tertiary filter design. The higher solids holding capacity of larger media (i.e., the Solids Capture Ratio) was effectively demonstrated in both studies, with an average value of 0.19 lb. TSS/ft2/ft of headloss achieved in the Henrico County pilot study. The effectiveness of MULTIWASH simultaneous and sustained air/water backwash to clean filter media resulting in extended filter run lengths was also demonstrated in both pilot studies.
Sand media and dual media did not produce the results desired because the filter run lengths and backwash recycle percentages were not of acceptable values. Coagulation with Alum and possibly with a filter aid (such as polymer) can improve effluent TSS removal and increase the solids capture ratio of the filter media.
Based on the pilot testing both Henrico County and Mt. Crawford wastewater treatment plants both selected large anthracite media for full scale system design. Both installations benefit from the pilot studies with regard to TSS removal efficiencies, filter run lengths and percent backwash recycle.