A process plant cannot function without water-based utility systems. While the importance of these systems isn’t usually contested, expenditures to expand or upgrade these operations often are avoided because no direct payback can be assigned to any utility capital expenditures. However, four major drivers are spurring increasing interest in optimizing water use:
1. Higher water-use demands and flows;
2. Water-utility-cost increases;
3. Wastewater-discharge-cost rises; and
4. More-stringent regulatory limits.
Higher water-use demands and flows. Increased demands can push the hydraulic limitations of the existing water-treatment, steam-production and wastewater-treatment equipment and decrease performance. Operating costs will likely rise due to the additional stress placed on the equipment while capital spending may be necessary to increase capacity to meet the demands.
Water use at a plant can increase for many reasons. Plant expansions and unit conversions can impact utility systems by boosting flows and contaminant loading. In addition, new and modified units may contribute additional stormwater runoff. Tighter product specifications also can lead to increased water demands from, for instance, additional washing steps. Another common source of higher demand is aging or failing equipment. Heat exchangers with leaking tubes that have been plugged or clogged tubes, for example, may require more water to meet cooling demands. Often external cooling from hoses serves as the supplemental cooling source.
Water-utility-cost increases. The cost of supplying water for steam, cooling and processing varies extremely depending on the water source. Water typically comes from sources such as on-site groundwater wells, surface water or an off-site provider. These supplies often have flow limit restrictions and purchased water costs can be prohibitive. There also may be additional regulations enforced when demands exceed certain permit limits. Costs for raw-water treatment (chemicals, sludge disposal, pumping, etc.)rise with demand.
Wastewater-utility-cost rises. There is a direct relationship between water demand and flows to wastewater treatment. Many wastewater-treatment units are designed for peak flows only experienced during storm conditions. Treatment costs during these peak flow conditions can climb exponentially due to increased pumping, aeration demands, sludge management and solids disposal requirements. Most importantly, extra water use cuts into treatment capacity to handle peak flows and often results in the need for additional storage capacity to dampen the peaks. Plants that discharge their wastewater stream to an offsite treatment authority are generally billed for usage based on metered flows — thus higher wastewater flows could represent a significant cost burden.
More-stringent regulatory limits. Regulations that can affect water-based utility system costs include vapor control limits (national emission control standards for hazardous air pollutants, etc.), wastewater discharge limits (National Pollutant Discharge Elimination System or state limits), and land disposal restrictions that regulate contaminants present in unlined storage/treatment ponds or collection systems. Control systems or equipment modifications designed to comply with these regulations often are sized based on flow or contaminant loadings. Increasing flows to existing treatment systems can lead to decreased performance if the systems either are not managed to accommodate these changes or cannot meet these new demands. For example, higher loading on a biotreatment system can result in the existing aeration system not meeting the new oxygen demand or in significantly lower hydraulic residence time or mixing capabilities, both of which can decrease performance and possibly cause a discharge permit violation.
Water-use systems impact nearly all plant operations either directly or indirectly. Steam, for example, is a common heat source in tubular exchangers. In reactor systems it can serve as a heat sink, as an inert diluent or for mixing. It also is used for duties as diverse as powering turbines, keeping instruments cool in an incinerator and as a barrier fluid in the seals of a compressor. Steam condensate and boiler feedwater provide a source of clean low-dissolved-solids water not only for boilers, but for makeup for chemical solutions, pump seal fluid or wash water for removal of precipitated salts. Cooling water goes to users ranging from large tubular exchangers to air conditioning units and sample coolers. Figure 1 shows the water balance for a typical plant.
Aqueous utility streams (steam, boiler feedwater, cooling water) pick up contaminants as they circulate throughout the system and when they contact process materials. Eventually these streams must be discharged to the wastewater-collection system for treatment. Some of the most significant contributors of flow and contaminants to wastewater treatment in many plants are:
• Raw-water treatment streams, e.g., sand filter backwash, reverse osmosis reject, and regeneration from deionization;
• Blowdown streams from cooling towers, boilers and process steam generators;
• Unrecovered condensate and cooling water; and
Figure 2 shows typical water users and contributors to wastewater-treatment loads.
The major players
As water demands rise, flows to wastewater treatment increase proportionally unless water reuse and recycle is boosted. Therefore, it makes sense to focus on the major contributors of wastewater flow and contaminants for potential opportunities to reduce, reuse, recycle or eliminate these streams.
Generally water requiring either discharge or treatment can be classified into four types: process wastewater, non-process wastewater, maintenance wastewater and stormwater. Process wastewater consists of streams that directly contact other unit process streams and that, therefore, may require controls for volatile emissions and treatment prior to discharge. Non-process wastewater consists of streams that don’t directly contact other unit process streams. Non-process streams typically contain contaminants at concentrations below all regulatory triggers and may be recovered for re-use or subject to less-stringent treatment requirements. Stormwater is defined as runoff collected throughout the site. Runoff from non-process areas is generally suitable for direct discharge via stormwater outfalls. Maintenance wastewater is generated during activities such as pump, heat-exchanger and instrument/analyzer clearing, filter changes, equipment washdown, deluge system testing, winterization and turnaround. Maintenance streams, depending on regulations and their potential contamination, often are subject to controls similar to those applicable to process streams.