Treatment system

Vapor Treatment. The vapors from the tank space can be sent to a treatment system (condenser, absorption, etc.) before venting. The system shown in Fig. 9.1 uses a vacuum-pressure relief valve which allows air in from the atmosphere when the liquid level falls (Fig. 9.1a) but forces the vapor through a treatment system when the tank is filled (Fig. 9.16). If inert gas blanketing is required, because of the flammable nature of the material, then a similar system can be adopted which draws inert gas rather than air when the liquid level falls. 

Figure 9.1 Storage tank fitted with a vapor treatment system. (From Smith and Petela, The Chemical Engineer, no. 517, 9 April, 1992 reproduced by permission of the Institution of Chemical Engineers.)...

As with safety, environmental considerations are usually left to a late stage in the design. However, like safety, early decisions often can lead to difficult environmental problems which later require complex solutions. Again, it is better to consider effluent problems as the design progresses in order to avoid complex waste treatment systems. 

The process is designed from a knowledge of physical concentrations, whereas aqueous effluent treatment systems are designed from a knowledge of BOD and COD. Thus we need to somehow establish the relationship between BOD, COD, and the concentration of waste streams leaving the process. Without measurements, relationships can only be established approximately. The relationship between BOD and COD is not easy to establish, since different materials will oxidize at different rates. To compound the problem, many wastes contain complex mixtures of oxidizable materials, perhaps together with chemicals that inhibit the oxidation reactions. 

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. 

Sludge disposal typically can be responsible for 25 to 40 percent of the operating costs of a biological treatment system. Treatment of sludge is aimed primarily at reducing its volume. This is so because the sludge is usually 95 to 99 percent water and the cost of disposal... 

All process Hcensors also feature wastewater treatment systems. Stamicarbon guarantees the lowest NH —urea content and has plants in operation confirming the low NH —urea (1 ppm NH —1 ppm urea). This water is very satisfactory to use as boiler feed water. See Figures 16 and 17 for this system. 

The function of aeration in a wastewater treatment system is to maintain an aerobic condition. Water, upon exposure to air, tends to estabUsh an equihbrium concentration of dissolved oxygen (DO). Oxygen absorption is controlled by gas solubiUty and diffusion at the gas—hquid interface. Mechanical or artificial aeration may be utilised to speed up this process. Agitating the water, creating drops or a thin layer, or bubbling air through water speeds up absorption because each increases the surface area at the interface. 

Several methods have been developed to estimate the oxygen demand in waste water treatment systems. Commonly used laboratory methods are biochemical oxygen demand (BOD), chemical oxygen demand (COD), total oxygen demand (TOD), total organic carbon (TOC), and theoretical oxygen demand (ThOD). 

Oxygen is used in these microbiolreactions to degrade substrates, in this case organic wastes, to produce energy required for ceU synthesis and for respiration. A minimum residual of 0.5 to 2.0 mg/L DO is usually maintained in the reactors to prevent oxygen depletion in the treatment systems. 

Fig. 2. LP Oxo gas recycle flow scheme A, feedstock pretreatment B, reactor C, catalyst preparation and treatment systems D, condenser E, separator F,...

In order to conform to environmental quaUty guidelines, mills have installed a number of primary and secondary treatment systems to control... 

J. C. BuzzeU, Jr. and co-workers. Behavior of Organic Chemicals in the Mquatic Environment, Part III—Behavior in Merobic Treatment Systems (Activated Sludge), Association of Manufacturing Chemists, Washington, D.C., 1969, pp. 26—31. 

There are two reasons why the concentration of quaternaries is beheved to remain at a low level in sewage treatment systems. First, quaternaries appear to bind anionic compounds and thus are effectively removed from wastewater by producing stable, lower toxicity compounds (205). Anionic compounds are present in sewer systems at significantly higher concentrations than are cations (202). Second, the nature of how most quaternaries are used ensures that their concentrations in wastewater treatment systems are always relatively low but steady. Consumer products such as fabric softeners, hair conditioners, and disinfectants contain only a small amount of quaternary compounds. This material is then diluted with large volumes of water during use. 

Linear alkylbenzenesulfonate showed no deleterious effect on agricultural crops exposed to this material (54,55). Kinetics of biodegradation have been studied in both wastewater treatment systems and natural degradation systems (48,57,58). Studies have concluded that linear alkylbenzenesulfonate does not pose a risk to the environment (50). Linear alkylbenzenesulfonate has a half-life of approximately one day in sewage sludge and natural water sources and a half-life of one to three weeks in soils. Aquatic environmental safety assessment has also shown that the material does not pose a hazard to the aquatic environment (56). 

Health nd SMety Factors. The lowest pubhshed human oral toxic dose is 430 mg/kg, causing nervous system disturbances and gastrointestinal symptoms. The LD q (rat, oral) is 750 mg/kg (183). Thiocyanates are destroyed readily by soil bacteria and by biological treatment systems in which the organisms become acclimatized to thiocyanate. Pyrolysis products and combustion products can include toxic hydrogen cyanide, hydrogen sulfide, sulfur oxides, and nitrogen oxides. 

Dissolved Air Flotation. Dissolved air flotation (DAF) is used to separate suspended soflds and oil and grease from aqueous streams and to concentrate or thicken sludges. Air bubbles carry or float these materials to the surface where they can be removed. The air bubbles are formed by pressurizing either the influent wastewater or a portion of the effluent in the presence of air. When the pressurized stream enters the flotation tank which is at atmospheric pressure, the dissolved air comes out of solution as tiny, microscopic bubbles. Dissolved air flotation is used in many wastewater treatment systems, but in the United States it is perhaps best known with respect to hazardous waste because it is associated with the Hsted waste, K048, DAF flotation soflds from petroleum refining wastewaters. Of course, the process itself is not what is hazardous, but the materials it helps to remove from refining wastewaters. 

Many different factors influence the performance of biological treatment systems. Although each specific biological process has special requirements, factors common to biological processes, besides biodegradabihty, include organic concentration, temperature, pH, nutrients, and oxygen (aerobic or anaerobic). 

Catalytic Oxidation. Catalytic oxidation is used only for gaseous streams because combustion reactions take place on the surface of the catalyst which otherwise would be covered by soHd material. Common catalysts are palladium [7440-05-3] and platinum [7440-06-4]. Because of the catalytic boost, operating temperatures and residence times are much lower which reduce operating costs. Catalysts in any treatment system are susceptible to poisoning (masking of or interference with the active sites). Catalysts can be poisoned or deactivated by sulfur, bismuth [7440-69-9] phosphoms [7723-14-0] arsenic, antimony, mercury, lead, zinc, tin [7440-31-5] or halogens (notably chlorine) platinum catalysts can tolerate sulfur compounds, but can be poisoned by chlorine. 

Fig. 20. Process flow diagram for PACT wastewater treatment system.

Wastewater Treatment Systems, Peformance and Cost, Roy F. Weston, Inc., West Chester, Pa., 1977. 

Other Sanitizers and Treatment Systems. Other sanitizers and swimming pool treatment systems used to a limited extent are bromine, quats, ozone, ionizers, electrolyzers, uv, UV/H2O2, and Ag/H202. 

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