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Veolia North America

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Wastewater Treatment: Secondary

Equalization

The equalization step is used to reduce the aeration in the loading to the treatment process. The process can provide dampening for both hydraulic and concentration. The sizing of the equalization system is dependent on the variability on the influent wastewater. Typical equalization provides for 6 to 24 hours of retention time. One of the primary difficulties in the operation of the equalization system is short-circuiting and solid deposition. The best designs take this into consideration and provide some type of mixing in the process. If air is used for mixing, special attention should be given to the regulation and the treatment that occurs in the equalization process should be recognized.

Aeration

The aeration process is described as follows:

Historically the air was supplied using surface aerators that required wastewater depth of less than 20 feet. In the newer designs, blowers are used to supply the air to the biomass and also provide mixing. In general, the smaller the air bubble the higher the oxygen transfer. The deeper the tank the less air/ cubic foot is required. When the water depth exceeds 30 feet blower selection becomes more limited.

The final biological process used in the treatment of refinery wastewater is where the biomass is attached to a surface that may include fixed film reactors or membrane reactors. This process preceded the activated sludge process in the industry. The process is dependent on the biomass becoming attached to some type of media. The principal of the treatment process are the same as those described in the preceding text.

This process has been recently improved by incorporating filtration into the media, Biological Aerated Filter (BAF). BiostyrTM is the latest design of the BAF technology to be offered to the marketplace. It combines in a single structure a biological reactor for pollution reduction together with an effective filtration method. BAF technology is well proven with over 100 installations worldwide. Operating BiostyrTM sites now achieve a wide spectrum of effluent performance ranging form carbon (COD and BOD) reduction to full nitrification and de-nitrification. All of these occurring with suspended solids minimization. The process is able to operate at such high removal rates (ammonia removal for example up to 60lb N-NH4/1000ft3/day) compared to traditional activated sludge. In addition, the required footprint is significantly smaller. The totally enclosed reactor provides one additional benefit, the elimination of odor issues commonly occurring with other technologies.

The BiosepTM is another advanced treatment process combining biological treatment in addition to the separation of sludge / water, all within one tank.

The separation is performed by submerged hollow fiber membranes. BiosepTM replaces the traditional treatment processes in the activated sludge tank, settling tank and post-filtration and is a very compact and efficient treatment process.

It is superior to other treatment processes regarding removal of organic matter, suspended matter, micro-organisms or nutrients and can fulfill the highest treatment demands.

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Activated Sludge

There are several types of biological treatment. These include aerobic, anaerobic and facultative. The microorganisms can be suspended in the liquid or attached to the surface of a media and the operation can be continuous or batch. The most common type of treatment is aerobic and will be the focus of the remainder of this discussion.

The most common type of biological treatment is activated sludge. The activated sludge process is dependent on recycling the acclimated biomass to allow contact with the wastewater in the presence of oxygen." In the activated sludge process, organic matter is removed from solution by biological metabolism, oxygen is consumed by the microorganisms and new cell mass is synthesized. The microorganisms also undergo progressive auto-oxidation (endogenous decay) of their cellular mass. When a wastewater is mixed with acclimated biological sludge, there may be an immediate "sorption" of readily degradable organics. These organics are stored within the cell for subsequent oxidation. This phenomenon is called bio-sorption. As aeration proceeds, removal of the remaining organics occurs. The oxygen uptake rate is initially high as the "sorbed" organics are degraded and then decreases as the residual substrate decreases. Cellular synthesis occurs in proportion to the organic removal. Nitrogen and phosphorus are taken up for cellular synthesis. When the available substrate is exhausted, continued aeration results in oxidation of the biomass through endogenous respiration.

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Sludge Dewatering

The type of press selected depends on the volume and characteristics of the sludge, the dryness of the cake and the method of disposal. Primary sludge is typically more difficult to dewater than the solids generated in the biological process.

If a centrifuge is used, a three phase separations is possible water, oil and solids. The water can be discharged to the activated bio treatment plant, oil can be returned to the refinery for processing and solid can be used as Coker feed, fuel, or sent to disposal.

Bio solids are generally dewatered in a plate and frame or and belt filter press. The disposal cost and liquid content in the cake dictate the equipment selection. A plate and frame press will provide a dryer cake in the range of 30 to 50 percent. A belt filter press will generally dewater the cake to a 15 to 20 % dry cake. The water or pessate is returned to the biological treatment plant and the solids are generally disposed of in a land fill.

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