The Use of Non-flowing Rinsing Tanks for the Restriction of Water Consumption

摘要

Replacement of flowing rinses by non-flowing ones with periodic dumping of the latters is one of the ways for restricting water consumption. In such an ease every rinsing tunk once filled with fresh water is operating without water exchange over a certain period of time and is dumped only, when the concentration of contaminants reaches certain critical value. After dumping the tank is filled with fresh water and the whole cycle is repeated again. Considerable reduction effect in water consumption can be reached only, if the number of such tanks equals at least two. Single flowing rinses, multistage concurrent and countercurrent rinses and reclaim tanks have been considered. If a single rinsing tank is attached to a process tank, its conversion to non-flowing rinse is not suitable. Table 1 and 2 contain data for the calculation of the duration of a single cycle of a continuous operation of two- and three-tanks system between the dumpings. The volume of each tank was taken equal to 1000 l(V_(Пр)), the surface area of treated parts per hour was taken equal to F m~2/hr, the specific dray-out (q) was assumed to be 0,2 l/m~2. Concentration of a component in process solution is designated by C_o and its limiting admissible concentration - by C_П. Water consumption per hour for rinsing is found as a ratio of the total volume of all rinsing tanks and the duration of a single cycle. In particular cases, when the actual values of C_o, V_(Пр), F and q do not coincide with those given in the tables, the latters are corrected to fit the actual situation using eq. 1. An example of the operation of the rinsing tanks under non-flowing conditions is given for the cyanide zinc-plating process (C_o 50 g/l; C_(П)0,01 g/l; q 0,31/m~2; F 1,5m~2/hr). The duration of the dumping cycle in this example is equal to 31 hr for the two rinsing tanks with V_(П) 7001 each and 169 hr for 3 similar tanks. Thus, in case of 2 tanks this cycle corresponds to 4 days and for 3 tanks it corresponds to 1 month which is very convenient from the point of view of water consumption regulation. Such a method of the regulation does not depend neither on the availability of water-controlling equipment, nor on the pressure fluctuations in the water-supplying system. The above-described approach to water-consumption regulation leads, obviously, to the regulation of the amounts of disposed liquid wastes, and long periods of operation between dumping mean higher concentrations of dissolved compounds in the waste water. A combination of these two conditions (low volumes and high concentrations) allows to create a new form of operation of waste-treatment facilities and utilize or even recuperate and return to the plating process certain valuable and high-toxic components (precious metals, chromium (Vl)- and nickel-compounds, etc.). And finally, if only deionized water is used in the plating and rinsing processes, the installation of a cascade of more than 4 non-flowing rinses allow to create a completely closed-loor process cycle.