Primary treatment of wastewater

Wet Air Oxidation. With wet air oxidation, increased temperature and pressure are used to oxidize dilute concentrations of organics and some inorganics, such as cyanide, in aqueous wastes that contain too much water to be incinerated, but are too toxic to be treated biologically. In general, wet air oxidation provides primary treatment for wastewaters that are subsequendy treated by conventional methods. This technology can be used with wastes that are pumpable (slurries andUquids). 

The vendor claims that Ensol is applicable for the treatment of wastewaters including sewage after primary treatment, metal plating and finishing waters, and paper mill white water. The claim also extends to the treatment of solid wastes and soils, provided that they contain sufficient free water. 

Current processes for the treatment of wastewater may be divided into three main categories of primary treatment, secondary treatment, and tertiary or advanced treatment (Figure 7). 

General discussions on the applications of filtration to wastewater flows were given in previous chapters. These discussions have provided the reader with an understanding of the objectives of wastewater treatment and how filtration can be used as one of the unit operations for solids removal. Filtration must, however, be used in conjunction with other treatment technologies or operations, and does not necessarily represent the primary treatment method. It is beyond the scope of this volume to discuss the other techniques used in the treatmait of wastewater flows. For general information and backgroimd on other unit operations and processes used with filtration in the treatment of wastewaters, the reader may refer to the suggested references at the end of this chapter. 

The plant has two primary sources for wastewater, the decanter (0.20 kg water/s) and the pot (0.15 kg/s). The plant has been shipiHDg the wastewater for off-site treatment. The cost of wastewater transportation and treatment is 0.01 kg leading to a wastewater treatment cost of proximately SltO.OOO/yr. The plant wishes to stop offsite treatment of wastewater to avoid the cost ( 110,000/yr) and alleviate legal-liability concerns in case of transportation accidents or inadequate treatment of the wastewater. The objective of this problem is to eliminate or reduce to the extent feeble off-site wastewater treatment. For capital budget authorizatitHi, the plant has the following economic criterion (see Appendix DI) ... 

Comminution. Mechanical shredding or pulverizing of waste a process that converts waste into homogeneous and more manageable material. It is used in solid waste management and in the primary stage of wastewater treatment. 

Primary or pretreatment of wastewater prior to biological treatment involves both physical and chemical treatment depending on the nature of the emission. 

The capital cost of most aqueous waste treatment operations is proportional to the total flow of wastewater, and the operating cost increases with decreasing concentration for a given mass of contaminant to be removed. Thus, if two streams require different treatment operations, it makes no sense to mix them and treat both streams in both treatment operations. This will increase both capital and operating costs. Rather, the streams should be segregated and treated separately in a distributed effluent treatment system. Indeed, effective primary treatment might mean that some streams do not need biological treatment at all. 

Low concentrations of oil can be removed by dissolved air flotation (DAF). In this process, an effluent recycle is pressurized in the presence of excess air, causing additional air to go into solution, in accordance with Henry s Law. When this water is discharged to the inlet chamber of the flotation unit at close to atmospheric pressure, the dissolved air comes out of solution in the form of tiny air bubbles which attach themselves to and become enmeshed in suspended solids and oil globules. The primary design criteria is the air/solids ratio, which is defined as the mass of air released divided by the mass of solids fed. Sufficient air must be released to capture the solids in the influent wastewater. The performance of DAF for the treatment of several... 

Secondary Treatment A wastewater treatment process used to convert dissolved or suspended materials into a form more readily separated from the water being treated. Usually the process follows primary treatment by sedimentation. The process commonly is a type of biological treatment process followed by secondary clarifiers that allow the solids to settle out from the water being treated. Sedimentation The process of subsidence and deposition of suspended matter from a wastewater by gravity. 

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