Background

Because of the downturn of the economy during 1991, the Environmental Control Department (ECD) at Miles Inc. began looking for ways to reduce variable costs within its department [12]. Utilizing a breakdown of departmental costs, the variable expenses attributed to the neutralization of wastewater were identified as an area of major concern (Figure 3).

This wastewater, which has an average pH less than 2, must be treated to meet discharge standards as set by the West Virginia Department of Natural Resources (WVDNR). The WVDNR issues permits that regulate wastewater discharges and require constant monitoring of the plant effluent pH. Adjusting the wastewater to pH 7 would be similar to raising the pH of vinegar (acidic) to that of tap water (neutral).

Consider the history of wastewater neutralization at this site. In the 1970s and early 1980s, lime was used as the primary neutralizing agent for wastewater. However, lime loses its efficiency as a neutralizing agent as the pH approaches 7, and any excess or unreacted lime settles out during clarification. (See Figure 4). Due to the organic compounds in the wastewater, the resultant sludge is classified as a hazardous waste and must be disposed of according to RCRA hazardous waste regulations.

In 1986 the switch was made to caustic soda as the principal neutralizing agent in an effort to reduce sludge generation rates. This system was very effective in limiting the amount of

Figure 3 General cost breakdown. Caustic soda used to neutralize the wastewater is seen to represent the greatest cost factor.

Lime Efficiency/Sludge Generation 50%

Figure 4 Lime efficiency/sludge generation versus pH.

100%

Figure 4 Lime efficiency/sludge generation versus pH.

wastewater sludge produced, but in 1990 caustic soda experienced a 52% increase in price, which dramatically increased the variable cost of neutralization.

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