Appropriate measures for recycle or reuse of wastewater differ depending on the specific situation. One feasible option often employed for recycling is to group streams based on TDS or contaminant content, e.g., recycle of low-TDS sour-water-stripper bottoms (in preference to high-TDS cooling tower blowdown). Specifications for water to be reused must be based on solid justification — however, being overly conservative drives up pretreatment cost and limits possibilities for reuse.
Operating practices can tax water-use utility systems and increase flows to wastewater treatment — and often provide promising opportunities to save water through installation of additional equipment, modifications to systems or operator training/awareness. The table lists some examples of common water-use practices that should evaluated for reduction or elimination.
Cut peak flow
While direct discharge of stormwater runoff from non-process areas usually is acceptable, runoff from process areas generally must either be treated or, less commonly, sampled prior to direct discharge. Permit conditions may also allow for either direct discharge following collection of first flush volumes or decreased treatment requirements of segregated stormwater streams. Options to reduce the impacts of stormwater on wastewater collection and treatment include:
• Covering process areas and routing segregated non-contact stormwater to direct discharge;
• Segregating non-process-area stormwater and sending it to direct discharge;
• Implementing first flush for management of stormwater — this requires a segregated collection and storage capacity system;
• Boosting storage capacity;
• Increasing the permeability of non-process areas to reduce total runoff; and
• Decreasing, if possible, the size of runoff collection zones in process areas.
Storage tanks can be used to lower wastewater-treatment peak hydraulic loadings, which dictate equipment sizing. The use of tanks also allows for better management of biotreatment systems, which do not effectively respond to sudden changes in flow and contaminant loadings.
Segregate clean streams
Many non-process streams are eligible for direct discharge depending on the governing discharge-permit requirements. Potential streams that should be considered for direct discharge include:
• Steam blowdown;
• Groundwater seepage;
• Fire water, potable water, clarified water, hydrotest water and non-contaminated construction water;
• Non-contact cooling water and condensate;
• Cooling tower blowdown;
• Steam tracing condensate; and
• Condensate and cooling water from heating/ventilating/air-conditioning units.
Other non-process streams may be considered for direct discharge if sufficient data and process knowledge are available to assure the discharge permitting authority that the streams are non-contaminated.
Reducing flows of non-oily wastewater obviously will lower the hydraulic loading of the collection and treatment system, which in turn should improve operations. However, such a move offers other benefits, too.
First, most hydrocarbon species have a limited solubility in water. By introducing non-oily wastewater into an oily wastewater stream that contains free oil, additional hydrocarbon will enter into solution based on saturation of the previously non-oily water with sparingly soluble hydrocarbon species. This has the effect of significantly increasing the soluble chemical oxygen demand (COD) load at treatment (generally biotreatment is used to remove soluble COD). The impact is most apparent when a low flow stream containing free hydrocarbon is mixed with a high flow clean stream.
The second reason for segregating clean streams from oily ones (process wastewater) is that some of the large flow non-process streams contain solids or form fine precipitates when mixed with other streams that are at a higher pH. Examples include blowdown from cooling towers and boilers. The solids and fine precipitates combine with hydrocarbons that may be present in the process wastewater to form emulsions. These emulsions can be difficult to remove with separation technologies (separators, air flotation units, etc.), thus increasing the hydrocarbon load to secondary treatment (typically biotreatment).
Recycle and reuse
The reuse of treated effluent does not reduce the total flow rate of wastewater requiring treatment and thus, is generally less desirable than the direct reuse of clean wastewater. This option does, however, decrease water demands.
One of the more-common unit operations for wastewater treatment at process plants is biotreatment, which is effective for decreasing soluble organic concentrations. Reuse of effluent from biotreatment systems may require reducing solids concentrations through filtration or other technologies; residual organic concentrations must be low to minimize the need for pretreatment before reuse. Such treated effluent often is suitable for washwater from hose stations and other uses that can tolerate relatively high dissolved solids content. Removing the dissolved solids often opens up other opportunities such as cooling tower makeup, boiler feedwater makeup, process water or once-through cooling water but adds to cost and reduces operability in that the reused water supply is only available when the dissolved solids removal equipment is online.
Significant opportunities generally exist to reduce the total load on the water-use system (Figure 3). Employing measures to recycle or reuse streams, eliminate or reduce water-use-intensive practices, decrease stormwater impacts, and segregate clean streams for direct discharge will help to optimize the utility system. They may lead to reduced capital costs for upgrades and expansions and lower operating costs, as well as improved treatment performance and in turn fewer discharge permit violations.
Tabatha Pellerin, P.E., is a process engineer with ENSR International, Westford, Mass. E-mail her at email@example.com.
John Woodhull, P.E., is the manager of process engineering for ENSR International, Westford, Mass. His e-mail address is firstname.lastname@example.org.