Alkaline waste tanks, treatment

Corrective Action Application At a hazardous waste treatment storage and disposal facility in Washington State, a cyanide-bearing waste required treatment. The influent waste stream contained 15 percent cyanide. Electrolytic oxidation was used to reduce the cyanide concentration to less than 5 percent. Alkaline chlorination was used to further reduce the cyanide concentration to 50 mg/1 (the cleanup objective). The electrolytic process was used as a first stage treatment because the heat of reaction, using alkaline chlorination to treat the concentrated cyanide waste, would be so great that it would melt the reactor tank. 

Sour effluents need to be neutralized under all circumstances before they are allowed to enter the activated sludge tank, whereas an alkaline waste water inrush rarely disturbs the system because the steady production of CO2 as a consequence of microbic activity tends to neutralize the system automatically. Toxic substartces have to be eliminated by a specific pretreatment. Flocculation and adsorption processes have been found suitable for this purpose. Under special circumstances, the respective effluent stream may even have to be branched off for separate treatment. 

The acid and alkali wastes are pumped from the acid-alkali wastewater sump [T-30] into the acid-alkali treatment module [T-31], Metering pumps controlled by pH instruments feed either acid or caustic to the module as required to maintain an acceptable alkalinity for the formation of metal hydroxides prior to discharge to the precipitator consisting of a mixing tank [T-98], a surge tank [T-99], and a sedimentation clarifier [T-101], The pH is adjusted to a value of 8.5 for optimum metal hydroxide formation and removal. 

In normal operation, the PRF generates about 120 m3 of salt waste solution per month. Currently, this waste solution is made alkaline and routed to underground storage tanks where it mixes with other Hanford defense waste liquors. An alternative waste treatment scheme is desirable to avoid converting large volumes of non-actinide waste to retrievable actinide waste (>10 nCi alpha activity/g) and also to help make the PRF independent of future tank farm management operations. 

---