• 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.
Non-process streams frequently account for a majority of flow to wastewater treatment. Data from numerous facilities show that these streams can account for well over 50% of the total flow to wastewater treatment. Stormwater also can have a huge impact on wastewater hydraulic loading because runoff rates often set the instantaneous peak flow rate to waste treatment. It is not unusual for peak runoff rates to exceed average wastewater flow rates by a factor of 100, particularly at a Gulf Coast location. Both of these “clean” streams not only contribute to the hydraulic loading at wastewater treatment but also affect treatment performance.
Wastewater treatment capacity is tied to volumetric flow rate and contaminant mass loading, both of which are likely to increase following significant modifications such as plant expansions or unit conversions. In such situations, most facilities undertake a series of wastewater treatment modifications and make any cost-effective changes to the treatment facilities. This leaves four approaches as the only viable options for matching treatment system capacity with wastewater load:
1. Reduce need for water-based utilities;
2. Decrease peak flow from stormwater;
3. Segregate clean streams for direct discharge; and
4. Recycle/reuse treated wastewater.
Decreasing demand is almost certain to result in a corresponding cut in wastewater volume and potentially less contaminant loading.
Non-process sources, stormwater and many common process wastewater sources can effectively serve as substitute supplies in many applications. Success depends upon carefully matching the water quality requirements with the characteristics of the wastewater. It is necessary to take into account any impact that water reuse may have on the quality of the wastewater. Build-up of contamination within a recycle loop can occur. Also, water reuse can affect the quality and characteristics of the remaining wastewater that is to be routed to waste treatment or to discharge.
Now, let’s look at some examples of potential sources to consider for reuse opportunities and common contaminants (limitations) of each.
• Blowdown streams. Recirculating-cooling-tower and boiler-feed streams build up dissolved solids, especially calcium and magnesium compounds, silica and other contaminants that are relatively insoluble in water. These compounds, when heated, tend to precipitate out of solution, causing scale and corrosion. Blowdown streams are necessary to maintain low levels of solids in the recirculating streams. Blowdown streams contain concentrated levels of total dissolved solids (TDS) and for this reason are not good candidates for reuse.
• Water-treatment regeneration streams. Ion exchange media (salt brine for softeners, acid and bases for deionizers) used to remove both suspended and dissolved solids generally require regeneration. This typically consists of backwash, regenerant introduction and fresh-water rinse steps. These wastewater streams often have high levels of suspended and dissolved solids and may require pH adjustment depending on the exchange medium. The high dissolved or suspended solids contents of these streams preclude most reuse options.
• Condensate. By returning condensate to the boiler for use as feedwater, facilities reduce the makeup source demands and lower regeneration flows because condensate does not typically require water treatment. Condensate streams have low TDS and often high heat-recovery potential.
• Unrecovered cooling tower water or once-through cooling water. Water that is not returned to the cooling towers for recirculation contributes to the blowdown flows. Lowering unintentional losses of cooling water allows the blowdown rate to be controlled, which then enables the operator to control cycles of concentration. Maximizing water returned to the once-through cooling water system reduces hydraulic loading to the wastewater collection and treatment system. Cooling water generally has low TDS. These streams, though, potentially may have elevated hydrocarbon concentrations if there is a heat exchanger leak or other abnormal situation.