Industrial wastewater treatment facilities that are nearing their capacity limit often have a relief option that typically is not available to municipal treatment facilities. That option is to reduce waste loads coming from the production areas by optimizing manufacturing processes so that less waste is generated per pound of product made. In making the decision between reducing waste loads and increasing treatment capacity, the facility owners need to identify and thoroughly assess each performance limiting unit process and then determine the capital and operating costs of adding the next increment of capacity. The net present cost of the expansion option (expressed as cost per quantity of capacity added) is then used as the standard against which the waste minimization options are evaluated. This paper presents a case study where this approach was applied at a large chemical manufacturing facility.
Municipal wastewater treatment facilities often find themselves dealing with capacity limitations as the population in their service area grows. Faced with this situation, the typical response has been to either add capacity to the existing treatment facility or to construct an additional treatment plant. Industrial wastewater treatment facilities often find themselves faced with a similar predicament as manufacturing at the site increases and the plant's treatment capacity is in jeopardy of being exceeded.
For many years, industrial facilities approached this situation in much the same way as their municipal brethren and pursued the capacity expansion approach. However, as the dual pressures of increased global competition and more stringent environmental regulations became a fact of doing business, industries found that they had to start considering other options to ensure that they were making the most cost-effective decision overall.
One option industries often have that is typically not available to municipal facilities is the possibility of exerting some control over influent organic loadings. Most manufacturing processes were developed over the years with relatively little thought to the amount of waste generated during production, generally because waste disposal was not a significant cost when the manufacturing process was first implemented. Even though it has been gradually changing over time, most process development engineers still do not place a lot of emphasis on waste minimization when they are designing and evaluating new processes. This is unfortunate because it is often more cost-effective to prevent production of waste at the source than it is to treat it at the end of the pipe.
In order to determine whether a capacity-constrained industrial wastewater treatment facility should pursue capacity expansion or waste minimization, a systematic process was needed to weigh the two options. Such a process was devised and implemented at Eastman Chemical Company's chemical manufacturing plant in eastern Tennessee. This paper provides an overview of the process that has been used to compare capacity expansion and waste minimization options at this facility.
LIMITING PROCESS IDENTIFICATION
The first step in the process is to identify the major unit operations that could be causing a capacity constraint or limitation to overall treatment. The initial tool in this step is the 'Performance Potential Graph' or PPG. The purpose of the PPG is to normalize the treatment capacity of each major unit operation in terms of organic load. At Eastman Chemical Company's Tennessee Operations facility, total organic carbon (TOC) load has been used as the primary measure of organic load for at least twenty years and was therefore selected as the key metric for the PPG.
Next, the most appropriate time period for assessing capacity of each unit process was established (such as maximum weekly loading, maximum monthly loading, etc.). For example, maximum weekly load was considered to be the appropriate metric for the aeration basins and the blowers because influent diversion capabilities at the plant could be used to endure a few days of organic loads exceeding the treatment plant's capacity. On the other hand, the maximum monthly load was considered to be the appropriate metric for the clarifiers and belt filter presses. It was estimated that excess biological solids could be stored up in the aeration basins and diversion basins for a couple of weeks to provide some relief to the clarifiers and belt filter presses but a month of such storage was probably the limit.
Then the current capacity of each major unit operation was determined based on industry design standards, nameplate capacity, or 'real-life' operating experience. At the Eastman facility, the major unit operations were rated as follows:
- Aeration basins at 35 lb TOC/day per 1000 ft3 (based on typical industry value of 60 lb BOD5/day and conversion to TOC for Eastman's wastewater)
- Blowers at 35,000 lb TOC/day per blower (based on historical performance data)
- Clarifiers at a solids loading rate of 45 lb TSS/day/ft2 surface area (based on historical sludge yield, flow split, and sludge inventory)
- Belt filter presses at a solids loading rate of 600 lb TS/hr/meter of belt width (based on historical sludge yield and sludge age)