Summary The performance evaluation of tertiary granular media filtration systems indicates that mechanical problems are common with these systems. These . problems are primarily responsible for the impaired quality of effluent from the filtration system and the increased maintenance requirements. It is not unusual for such problems to occur considering that tertiary filtration of wastewater is a relatively recent development as a treatment process. The performance of tertiary filtration systems could be significantly improved by incorporating certain modifications in the design of the system, and ensuring that systematic operation and maintenance procedures are followed. A summary of the major problems identified and suggested remedies is presented in Table 1. vvEPA United States Environmental Protection Agency August 1984 Tertiary Granular Filtration Problems and Remedies J/3 For Additional Information: EPA Offices: EPA-OWPO (WH-547) 401 M Street SW Washington, DC 20460 (202)382-7365/7368 EPA-MERL (443) 26 West StClair Street Cincinnati, OH 45268 (513)684-7613 ------- backwashing systems such as surface wash or air scour do not operate properly, the bed will not be thoroughly cleaned and will eventually become clogged. Mudballs are agglomerations of extraneous material which accumulate in the bed over extended periods of time due to improper backwashing. Consequently, the effectiveness of the filtering and backwashing operation is significantly reduced. Loss of filter media is caused by excessively high backwashing rates, uneven distribution of backwash water at the bottom of the bed, and incorrect se- quencing of the auxiliary backwash system. Media loss increases operating costs for media replace- ment. It also results in an increased rate of buildup of head loss in the bed in dual media filters due to the loss of the coarser media from the top layer of the bed. Gravel mounding refers to disruption of the support gravel at the bottom of the filter bed due to excessively high backwash rates. Improper design of the filter underdrain system causes migration of the filter media and clogging of the backwash nozzles. These nozzles and other parts of the under- drain system can also be corroded due to galvanic corrosion. Suggested Remedies In order to ensure that the filter is backwashed at the appropriate time intervals, the frequency of back- washing should be controlled on the basis of both head loss and fixed time interval. The correct rate and duration of backwashing are essential for proper filter system operation. Manufacturer's speci- fications and operating instructions should be closely followed. Settings of the rate and timer controls for backwashing should be regularly checked. Maintaining the correct operational sequencing is critical for effective backwashing. The wash water troughs should be maintained at the correct level to prevent short circuiting during the backwashing cycle. At a minimum, a monthly visual check should be made of each filter cell through a complete backwash cycle to ensure that the correct operational sequence is maintained. A maintenance schedule should be established and followed to ensure that the downtime due to mechanical problems in the backwashing system is kept to a minimum. The use of backwashing nozzles fitted with a pro- tective plate on top should be considered to avoid clogging of the nozzles. In order to prevent electro- lytic corrosion problems, compatible materials of construction should be used for backwash nozzles, underdrain support structure and the filter bed. Problem Filtration Cycle • Frequent clogging of media and buildup of excessive head loss. « Hydraulic surges in influent flow to filters. o Excessive filtration system downtime due to equipment problems. • Incorrect operation of the filtration system. Backwashing Cycle • Improper frequency of backwashing. • Loss of media during backwashing. •.Clogging of backwash nozzles due to migration of media. • Corrosion of backwash nozzles. Remedy « Multimedia filter should be considered. Design filters to operate either in parallel or series. Improve quality of influent to filter by incorpor- ating modifications to pro- cesses ahead of filter. Add a disinfectant, usually chlorine, to the filter influent periodically. Ensure proper rate and duration of backwashing. • Equalization facilities ahead of filter should be provided, • An adequate maintenance program should be estab- lished and followed. • Formal training of opera- tors on filtration technology and operation of the system should be provided. o Backwash frequency should be controlled by both predetermined head loss and fixed time interval. o Excessive backwashing rates should be avoided. Correct operational sequencing of backwash- ing operation should be ensured. Proper levels of washwater troughs should be maintained. • Use of nozzles fitted with a protective plate on top should be considered. • Use of compatible materials of construction for nozzles, underdrain support structure, and filter bed to avoid electrolysis and galvanic corrosion. Table 1 Tertiary Filtration Systems: Summary of Problems and Suggested Remedies ------- Tertiary Granular Filtration - Problems and Remedies The major problems experienced with tertiary fil- tration systems and the recommendations for im- proving their performance are discussed separately for the filtration cycle and the backwash cycle. Start Figure 3. Gravity Filter Backwash Cycle. Common Problems - Filtration Cycle One of the most common problems with the filtration cycle is frequent clogging of the filter media. This results in buildup of excessive head loss in the bed within a short period of time and consequently in- creases the frequency of backwashing. Media clogging is caused by solids carryover from prior treatment processes, microbial growth in the1 filter bed, and high concentration of oil and grease in the influent to the filter. Operating difficulties and poor effluent quality are often caused by hydraulic surges in the influent flow. Maintenance operations at many treatment plants are not adequate to keep the filter system functioning properly. Insufficient maintenance can result in ineffective instrumentation systems and inoperative valves, pumps and other process control devices. inadequate training of personnel for operation of tertiary filtration systems is common in many plants.. This lack of knowledge adversely affects filter per- formance since necessary process adjustments are not made under critical operating conditions. Suggested Remedies The problems associated with the filtration cycle can be reduced by implementing the following re- commendations: (1) judiciously select the type of filter media. Multi-media filters provide efficient utilization of filter depth, better response to high solids loadings and longer filter runs. Single media filters cost less but generally require more frequent backwashing than multimedia filters; (2) design the filter system with the option to operate either in parallel or series mode. In case of increased suspended solids loading, series operation could help to meet the requirements for effluent quality and minimize the problem of frequent backwashing; (3) incorporate modifications to the upstream clarifica- tion process (e.g., polymer addition) to decrease suspended solids loading in the clarifier effluent flowing to the filter; (4) add a ballast tank to stabilize the influent flow and solids concentration; (5) record influent flows to filters and provide an automatic controller or weir to ensure even flow distribution among filters. When flow exceeds the filter's design hydraulic capacity, provisions to divert the additional flow to stand-by filters or surge tanks should be considered; (6) apply a disinfectant, usually chlorine, to filters taken off-line temporarily, to control microbial growth in the filter bed; (7) provide pre- treatment process to remove oil and grease in the filter influent; (8) establish an adequate maintenance program for regularly checking the filtration equip- ment such as flow controller, media depth, valves and pumps; and (9) provide formal training on filtration technology and filter operation to all operators who work with the filtration system. Common Problems - Backwash Cycle Several problems are associated with the backwashing cycle of tertiary filtration systems. Media clogging, mudball formation, media loss, and gravel mounding and displacement are common problems of the backwashing cycle. Backwash systems designed to initiate operation based on a single criterion (i.e., either fixed time interval or head loss) can result in improper frequency of backwash.Media clogging results from solids buildup in the filter bed due to insufficient rate or duration of backwashing. If the auxiliary ------- Tertiary Granular Filtration - Problems and Remedies Introduction Tertiary granular media filtration is utilized in wastewater treatment for the removal of suspended and/or colloidal matter from effluents of biological and chemical treatment processes. The use of filtration systems for wastewater treatment is a relatively recent development. However, they have been used in water treatment for over 50 years. The concept of utilizing filtration in wastewater treatment came into practice to meet the more stringent re- quirements for effluent not possible with conven- tional secondary treatment. Tertitary filtration is primarily aimed at removing the fine suspended solids that are carried over in the effluent from the final clarification process. The BOD associated with the suspended solids is also removed during filtration. The resulting filtered effluent is low in BOD and suspended solids concentration and, in general, meets the effluent standards for advanced waste treatment. A comprehensive evaluation of selected advanced treatment (AT) facilities utilizing granular filtration. systems was recently completed wittvllje objective of identifying common problems assoBfated with tertiary filtration systems. The primary Impacts resulting from these problems are short filter runs, increased frequency of backwashing, poor effluent quality and high maintenance costs. Based on this evaluation, suggested remedies to the problems identified are offered. Process Description Filters are classified according to configuration of filter media, media depth, direction of flow, type of flow, and method of flow rate control. The three types of filter media configurations used with granular media filters are as follows: (1) single medium — usually sand, (2) dual media — usually anthracite and sand, (3) multimedia — usually an- thracite, sand, and garnet. A filter can be a "shallow" (2 to 3 feet) or "deep" (5 to 6 feet) bed type. Filters are designed for upflow or downflow through the filter bed. Filters can be of the gravity or pressure type, and can be operated at either a constant rate of flow or declining rate of flow. Typical granular media filter configurations are shown in Figures 1 and 2. Filters are periodically backwashed with clean water to remove the sus- pended solids that accumulate in the filter media. Backwash can be initiated either by a predetermined head loss or by a timer set at a fixed time interval. In the backwash cycle expansion of the bed occurs due to fluidization, and the suspended solids that accumulate on the grains of the media are scoured and removed by the shearing action of the water and interparticle collisions. Figure 1. Typical Gravity Type (Downflow) Granular Media Filter. Figure 2. Typical Pressure Type Granular Media Filter. In order to improve the effectiveness of the back- washing operation, auxiliary surface-wash or air- scour systems are often used. Figure 3 shows a typical backwash operation cycle for a gravity filter. . In addition to the filter media and backwash systems, several appurtenances are necessary for operation of the filter, including the following: (1) underdrain system, which collects,.the filtered effluent and distributes the backwash water, (2) w'ashwater troughs used to collect and remove the backwash water from the filter, and (3) surface-wash or air-scour systems, which are used as auxiliary aids to backwash systems. ------- |