Coagulant Injection Equipment Installation

If coagulant feed systems are used, they shall be installed with the injection point located before the filters as far ahead as possible, with electrical interlocks in accordance with MAHC Section 4.7.3.2.1.3.

To enhance filter performance, a coagulant feed system, when used, should be installed with an injection point located before the filters and, for pressure filters, on the suction side of the recirculation pump(s) capable of delivering a variable dose of a coagulant (e.g., polyaluminum chloride or a pool clarifier product) to enhance filter performance. Pumps should be properly sized to allow for continuous delivery of the recommended dosage of the selected coagulant. Products used to enhance filter performance should be used according to the manufacturers’ recommendations. The coagulant feed system should consist of a pump, supply reservoir, tubing, ISOLATION valve, and BACKFLOW prevention device. Sand filters used as pre-filters for membranes or cartridge filters with 1-micron nominal or 5-micron absolute size ratings or less should not be recommended to have coagulant injection equipment. Specialized granular filter media capable of removing Cryptosporidium OOCYSTS or an acceptable 4.5-micron surrogate particle with an efficiency of at least 90% (i.e., a minimum of 1 log reduction) without coagulation should not be recommended to provide coagulant injection equipment, but this media should be replaced or reconditioned as recommended to sustain the minimum recommended particle removal efficiency stated above. Sand filters located ahead of a UV or ozone DISINFECTION system may be excluded from supplying coagulation equipment with the approval of the local authorities. Local authorities should consider the efficiency of the supplemental DISINFECTION process for Cryptosporidium inactivation but should also consider that a side-stream system does not have any effect on the Cryptosporidium OOCYSTS that bypass the system on each TURNOVER. For example, a UV system that is 99.999% effective at inactivating Cryptosporidium that only treats half of the recirculated water flow is on average only 50% effective (per pass) because all of the Cryptosporidium in the bypass stream remain unaffected by the UV. Coagulation is the key to effective granular media filtration, which has long been recognized in the drinking water industry.²⁰²,²⁰³,²⁰⁴,²⁰⁵,²⁰⁶ Operation of granular media filters without coagulation is not permitted by U.S. EPA regulations for drinking water treatment, with the exception of slow sand filters. Thus, if pathogen removal is a goal of water filtration for swimming POOL sand filters, coagulation would be essential. This is the rationale for recommending future consideration of coagulation in swimming POOLS. A coagulant feed system should be installed with an injection point located ahead of the filters to facilitate particle removal by filtration (instead of settling to the bottom of the pool), and injection ahead of the recirculation pump(s) will provide mixing to evenly distribute the coagulant among the particles. A variable dose of a coagulant (e.g., polyaluminum chloride, or pool clarifier) is recommended because coagulant dosages may vary with BATHER LOAD. Products used to enhance filter performance should be used according to the manufacturers’ recommendations since overfeed or underfeed of coagulants is known to impair performance. Although polyaluminum chloride (PACl) is not a widely used coagulant in the U.S. at present, it has been used extensively abroad.²⁰⁷,²⁰⁸ However, recommended dosages abroad may not be optimized for pathogen removal. PWTAG recommends a polyaluminum chloride dosage of 0.005 mg/L as Al, but research has shown that 0.05 mg/L is recommended to exceed 90% removal and 0.21 mg/L or higher could be optimal with filters operated based on U.K. STANDARDS.²⁰⁹ New Challenges: The Impact of Coagulation on Backwashing Coagulation is likely to make cleaning of sand filters more challenging. Drinking water treatment facilities in the U.S. employ auxiliary backwash systems such as air-scour to improve the cleaning process. Using water alone for backwashing has not been found to be effective for media cleaning in drinking water treatment applications.²¹⁰ Air scour systems are common in European AQUATIC VENUE filters and should be investigated further in the U.S. More frequent backwashing is recommended with water-only backwash, and the clean-bed headloss (pressure) should be recorded after each backwash to detect early signs of ineffective backwashing and prevent filter system failures. Initial Headloss and Headloss Accumulation Rate Increased headloss (or pressure buildup) in filters is expected with coagulation as particles are likely to be removed faster (more efficiently) and closer to the top of the filter thereby clogging the top of the filter more quickly. This is actually a sign that the coagulation/filtration system is working effectively. The initial headloss after backwashing should remain relatively constant however. Coagulants have been used successfully in the U.S. in the past and are currently being used in POOLS abroad. ²¹¹,²¹², ²¹³,²¹⁴ In systems not properly designed to backwash with filter effluent from other filters, the coagulant feed system should not be operated during backwashing so as to prevent introduction of coagulant into the backwash water.