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In 1990, the Regional Board gave treatment plants in the Upper Santa Ana River Basin new nitrogen limits. Initially the limit for total inorganic nitrogen (TIN) was 22 mg/l. The TIN limit was subsequently reduced to 18.3 mg/l in 1990 and 12 mg/l in 1995. The TIN limit now
is 10 mg/l for tertiary effluent that is percolated into the groundwater. IEUA decided to try and meet the new nitrogen limits (the lemon) by modifying its operations rather than spending over 20,000,000 for new concrete tanks. The operational methods selected also reduced the energy consumption
and lowered operational costs (lemonade). The result of the operational changes was to obtain full nitrification and to reduce the effluent TIN from over 27 mg/l in 1990 to less than 8 mg/l in 1999. A new aeration system was then installed to gain back lost aeration capacity. Membrane
panels were selected because of their high oxygen transfer efficiency and because it was believed that membrane panels could transfer more pounds of oxygen per day per cubic foot of volume. The increased oxygen transfer would allow for aeration to occur in one half of the basin. This would
allow the other half of the aeration basin to be used as an anoxic zone. The anoxic-oxic-anoxic-oxic (A-O-A-O) process sequence was used. The design criterion for the membrane panel aeration system was to provide maximum panel density with maximum airflow per panel. Using this configuration
an effluent TIN of 8 mg/l was obtained. The MLSS was then increased to 4500-5000 mg/l in the summer and about 4000 mg/l in the winter. This allowed for maximum nitrification to occur in the smaller aerated or oxic section of the aeration basin and maximum nitrogen removal in
the anoxic zone. The higher MLSS concentrations resulted in higher solids loadings, which resulted in very high sludge blankets in the secondary clarifiers. It was necessary to keep the SVI's less than 130 to control the sludge settleability and keep the sludge in the secondary clarifiers
during high flows.
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