Page 187 - ro membanes

170 8. GRANULAR MEDIA FILTRATION
120e140 mg/L for alkaline soak. The target pH for the acid soak is 4 while that for the
alkaline soak is 11.
3. Let the filter cell soak in acid (or caustic soda respectively) for 16e24 h. Measure the pH of
the water every 6 h to and once the pH reaches 7.5e7.8, drain the cell and backwash it.
4. Send the filter effluent to waste for 30 min after the low- or high-pH soak.
The most common operational problems of granular media pretreatment filters and
proven solutions for maintaining a steady and consistent filter performance are summarized in Table 8.3.
TABLE 8.3 Granular Media FiltersdTypical Problems and Solutions
Indicator/Problem
• Reddish brown discoloration of SDI test pads used for filter effluent testing
• Black/dark brown discoloration of SDI test pads
• Solid particles observed on SDI test pads
• High frequency of backwashing (short cycles up to 2e6 h)
Potential Causes Solution
• Overdosing of iron-based coagulant • Reduce the coagulant dosing rate, by at least 50%
• Adjust coagulant dose by jar testing with addition of flocculant and pH adjustment
• Use of low-purity ferric chloride with • Change the coagulant and use ferric high content of manganese sulfate
• Check the ORP of water fed and permeate of all RO trains to avoid membrane oxidation
• Broken nozzles or cracks on the • Check underdrain system and repair bottom of filter cells or replace the faulty devices
• Intermixing of sand and anthracite • Replace both sand and anthracite media on the filter surface with media of adequate
specifications (avoid the use of overly heavy anthracite)
• Improper upstream coagulation • Check/adjust the coagulant/polymer • Polymer overdosing dosage
• Low efficiency of the backwash • Check/adjust backwash rates of procedure water and air according to
recommended good practicesdbackwash rate at least 2.5 times higher than filtration rate
• Nozzles clogging • Empty the filter and start a recovery cleaning procedure
• High content of fine silt in source • Check the filter surface after filtration water cycles for silt accumulation and apply preventive measures (see
below)
• Algae growth • Remove algae/debris from the filter • Surface clogging surface, add/increase the dose of
coagulant/flocculant