Effluent minimisation

Problems of removal of mercury from aqueous effluents are more comphcated in plants that manufacture a variety of inorganic and organic mercury compounds it is generally best to separate the effluent streams of inorganic and organic mercurials. When phenyhnercuric acetate is precipitated from its solution in acetic acid by addition of water, the filtrate is collected and reused for the next precipitation. This type of recycling is necessary not only for economic reasons but also to minimise recovery operations. 

The l-chloro-2-propanol isomer represents about 85% of the chlorohydrin produced. In order to minimise the formation of dichlotide coproduct and ether, the reactant compositions are chosen such that the effluent Hquid contains 4—5 wt % propylene chlorohydrin. Under these conditions, the yield of chlorohydrin, dichloride, and ether from the reactants is reported to be 87—90, 6—9, and 2%, respectively (109,110,112). 

Alternatively, all the bromine can be generated from H2O2 but adding HBr at the start of the reaction instead of bromine. In both cases the bromine is fully utilised and thus bromide effluent is minimised. (Scheme 1). 

The collection and the processing of the effluent gases for minimising environmental pollution is difficult and expensive. 

The objective function used in the mathematical model is either the maximisation of profit or the minimisation of effluent. This is dependent on the nature of the data given for a problem. If the production, e.g. number of batches of each product or total tonnage, is not given then the objective function is the maximisation of profit. However, if this is given then the objective function is the minimisation of effluent. 

The second illustrative example is a modified literature example. The example was originally presented by Kim and Smith (2004). The example involves 7 water using operations with three contaminants present in the system. The example was only solved considering a central storage vessel due to the fact that the schedule used by Kim and Smith was retained for this example and there are few direct reuse opportunities within the given schedule. Due to the schedule being known, the objective was to minimise effluent. 

Gouws, J., Majozi, T., 2008. Impact of multiple storage in wastewater minimisation for multicontaminant batch plants towards zero effluent. Ind. Eng. Chem. Res., 47 369-379 Majozi, T., Zhu, X., 2001. A novel continuous-time MILP formulation for multipurpose batch plants. 1. Short-term scheduling. Ind. Eng. Chem. Res. 40(23) 5935-5949 Quesada, I., Grossmann, I. E., 1995. Global optimization of bilinear process networks with multicomponent flows. Comput. Chem. Eng. 19 1219-1242... 

The methodology deals with two types of problems, namely, the wastewater minimisation problem within a given plant structure and the plant synthesis problem. Each of these is dealt with in the form of two mathematical formulations. The first mathematical formulation deals with the scheduling of an existing operation as to produce near zero effluent. The second mathematical formulation deals with the... 

The objective function can take two forms depending on the production information given. If the required production is known, then the objective is the minimisation of effluent. If this is not the case then the objective function takes the form of a profit function, where profit is dependent on the revenue from product, the cost of the raw material and the treatment costs of the effluent. 

The objective function used in the zero effluent synthesis formulation is the minimisation of cost. The cost function is defined in constraint (8.62). The cost in... 

The required production over a 36 h time horizon is 2 batches of product 1 and 3 batches of product 2. The objective function used for this example is the minimisation of effluent, since the total production is given. 

Two formulations were derived. The first deals with minimising the amount of effluent produced from an operation where wastewater can be reused in product formulation and the plant structure is known. The minimisation is achieved by scheduling the operation in such a manner as to maximise the opportunity for wastewater reuse. The second deals with the synthesis of a batch process operating in zero effluent mode. The formulation determines the number and size of processing and storage vessels as to minimise the cost of the equipment and the amount of effluent produced from the resulting operation, while achieving the required production. 

Gouws, J., Majozi, T., 2008. Impact of multiple storage in wastewater minimisation for multicontaminant batch plants towards zero effluent. Ind. Eng. Chem. Res., 47 369-379... 

The method presented below utilises inherent storage in idle processing units within a wastewater minimisation framework to allow for the minimisation of effluent and storage. The methodology can be used to either minimise the size of the central storage vessel or provide alternative storage opportunities in the wastewater minimisation problem. 

The second example was solved for two cases. In the first case wastewater storage was only available in the form of a central storage vessel, no inherent storage was included. In the second case wastewater storage was available in the form of inherent storage and a central storage vessel. In both instances the objective function was the minimisation of effluent, since the production was known. 

One of the earliest fibre pretreatments for improving the dyeability of cotton is of course mercerisation (section 10.5.4). However, more recent research interest in this area has been generated by environmental concerns about reactive dyeing, aiming to enhance substantivity for the modified fibre so that higher absorption and fixation are obtained. This results in less dye (hydrolysed or still active) in the effluent. A further objective is to minimise the usage of electrolyte in the application process. This area has been thoroughly reviewed [392,393]. 

Many wastewater flows in industry can not be treated by standard aerobic or anaerobic treatment methods due to the presence of relatively low concentration of toxic pollutants. Ozone can be used as a pretreatment step for the selective oxidation of these toxic pollutants. Due to the high costs of ozone it is important to minimise the loss of ozone due to reaction of ozone with non-toxic easily biodegradable compounds, ozone decay and discharge of ozone with the effluent from the ozone reactor. By means of a mathematical model, set up for a plug flow reactor and a continuos flow stirred tank reactor, it is possible to calculate more quantitatively the efficiency of the ozone use, independent of reaction kinetics, mass transfer rates of ozone and reactor type. The model predicts that the oxidation process is most efficiently realised by application of a plug flow reactor instead of a continuous flow stirred tank reactor. 

There is growing concern over the potential risks to human health and the environment arising from leather goods. Dye manufacturers and tanneries are concerned about effluent, air pollution, containers and packaging [56]. In the light of the relatively important contribution to leather dyeing of azo dyes that can yield hazardous arylamines on reduction, careful guidance on the selection of dyes for leather is essential, with emphasis on procurement from reliable sources and the utilisation of liquid formulations to minimise... 

Heavy metals are widely used as catalysts in the manufacture of anthraquinonoid dyes. Mercury is used when sulphonating anthraquinones and copper when reacting arylamines with bromoanthraquinones. Much effort has been devoted to minimising the trace metal content of such colorants and in effluents from dyemaking plants. Metal salts are used as reactants in dye synthesis, particularly in the ranges of premetallised acid, direct or reactive dyes, which usually contain copper, chromium, nickel or cobalt. These structures are described in detail in Chapter 5, where the implications in terms of environmental problems are also discussed. Certain basic dyes and stabilised azoic diazo components (Fast Salts) are marketed in the form of tetrachlorozincate complex salts. The environmental impact of the heavy metal salts used in dye application processes is dealt with in Volume 2. 

A low consistency suspension of fibres, pigments and chemical additives (thin stock) flows on to the moving wire mesh filtration medium of the paper machine where the wet web is formed. The water which drains away in this process (white water) is then recycled as far as it is practical to do so and is used to dilute the incoming high consistency suspension (thick stock). Retention can be considered either in overall terms or in terms of a single pass of the thin stock across the machine wire (first pass retention). For efficient operation, paper makers aim to achieve as high a first pass retention as possible, which they do by the use of retention aids. This reduces material losses and also assists in minimising the level of suspended solids in the effluent. 

---