Nitrate-nitrogen is reduced in the wastewater stream in the Denitrification filters. The nitrate-nitrogen is converted to nitrogen gas through a biological reaction known as denitrification. This takes place in the absence of dissolved oxygen and the presence of a readily biodegradable carbon source. This is known as an anoxic environment. There are numerous organisms capable of the facultative process of denitrification. The presence of these organisms should not be an issue. The presence of nitrate/nitrite-nitrogen is dependent on the level of nitrification occurring in the nitrifying trickling filters (NTF). Most of the biodegradable carbon is removed in trickling filter/solids contact and NTF processes. A supplemental carbon source must be added to accomplish denitrification. In this process, the carbon source is added in the form of 99% methanol. There is a ratio of 3:1 of methanol (mg/L) required for every mg/L of nitrate-nitrogen. However, care is taken not to overdose methanol as every 1.0 mg/L of residual methanol adds 1.5 mg/L of biochemical oxygen demand (BOD) to the effluent.
The effluent from the NTF flows to a pump station that feeds the denitrification filters through an influent channel. The eight filters are operated in a parallel configuration with each filter having the option of running in a dual operation of denitrification/deep bed filtration or solely as a deep bed filter. Each filter has two separate inlet channels with Cutthroat flumes to standardize flow rates. One of the flumes has a methanol feed diffuser for carbon addition prior to filter. For the filter to maintain a proper flow rate in denitrification mode, the gate is closed to the channel without the methanol feed and is open to the channel with the methanol feed. Both gates are opened when the filter is in the high rate filtration only mode. Only one gate is open for low rate filtration mode.
There are eight, deep bed denitrification filters at the facility which serve two purposes:
1. Removal of nitrate-nitrogen
2. Removal of suspended solids
Each filter is layered with specially sized and shaped granular media. The primary layer being composed of high grade silica sand, which promotes a fixed-film growth of the biomass for denitrification. In addition to denitrification the filter also acts as a deep bed sand filter for suspended solids removal. The layering of the filters is composed of 8' sand media on the top with five, 4" additional layers below, which increase in particle size to the bottom which is a coarse gravel. The filter media is supported from the bottom of the filter by a T- Block configuration. It is specially designed to prevent the media from passing through while allowing air and water to be pumped through in the opposite direction during backwash and bump cycles.
The bump cycle is specifically designed to release nitrogen gas that accumulates in the media, which decreases the capillary action of the filter and increases head-loss. The bump is an operation where the filtered effluent is pumped back through the media at a rate of 6 gpm/sq. ft. This shifts the media slightly which releases entrained nitrogen gas bubbles. A bump cycle is initiated by time and occurs once every two hours except during low flows of less than 10 MGD. The time between bump cycles is adjustable.
During normal filter operation , head-loss builds up as solids accumulate in the media from the filtration function and the growth of biomass. To mitigate this, the filter is designed to pump water in the reverse direction as is done in a bump cycle as well as aerate the filter at a rate of 5 CFM/sq ft. This backwash and aeration stirs up the accumulated solids which rise to the top from the aeration. A motorized valve opens during this process and the waste stream flows by gravity to the mudwell. As the mudwell is filled solids are then pumped to the plant headworks. The backwash cycle can be initiated by four methods:
1. Flow Controlled
2. Level Controlled
3. Time Controlled
The backwash frequency must be carefully selected as too frequent causes a waste of methanol due to the fact that it is the methanol that reduces the dissolved oxygen after a backwash thus the fewer backwash cycles the less methanol is wasted. If the backwash cycles are not frequent enough, it will have an effect of the volume of water that is filtered and could impact water quality. The backwash water supply is derived from the stored filter effluent in the clearwell.