By choice or mandate, owners and design engineers for many new power and industrial plants are selecting alternatives to fresh water for plant makeup. An increasingly common alternative is effluent from a publicly owned treatment works (POTW), i.e., municipal wastewater treatment plant. These waters typically contain elevated concentrations of ammonia, nitrite/nitrate, organics, phosphate, surfactants, and suspended solids, all of which, if left untreated, can lead to serious microbiological fouling issues in cooling towers and cooling systems.
Fouled cooling tower film fill
However, with proper pretreatment and cooling water chemical treatment, such supplies can successfully be utilized.
The table below outlines the primary constituents from several different water sources around the United States.
Table 1. Comparison of Several U.S. Waters
(Values apart from pH and Conductivity are in units of mg/L)
As can be seen, grey water (also known as treated wastewater effluent or reclaim water) contains higher concentrations of many impurities.
Basic clarification is a common technique for initial treatment, but for the most part, it only removes suspended solids and some large organics. Using an aluminum- or iron-based coagulant precipitates phosphate and can therefore be a useful form of basic clarification. Like other technologies, clarifier design has been significantly enhanced over the large, low-flow rate circular types that were ubiquitous during the last century. Now, in systems such as the ballast type (typically micro-sand or magnetite), flow rates per clarifier capacity may be 10–20 times greater than in the old styles.
However, clarification does nothing to remove soluble nitrogen-based impurities, particularly ammonia and nitrite/nitrate. Many organics can also carry through a clarifier. Thus, even with clarification for suspended solids and phosphate removal, much food/nutrients can still enter the plant. These issues may be mitigated by selecting biological processes for pretreatment.
Biological methods to treat both municipal and industrial wastewater have been utilized for many years. Very common is the activated sludge process, in which the waste stream flows into a large basin or basins filled with beneficial microorganisms that consume the organics and nitrogen/phosphorus nutrients. The term “activated” comes from the fact that air is injected into pond volume, often at numerous locations, to provide an aerobic environment for the microbes, transforming soluble material in new cells that can be removed as suspended solids. A common variation on this technology is the trickling filter, in which the wastewater flows over the fixed media to which the organisms are attached, providing a stable base for the microbes to carry out waste removal.
A difficulty with these technologies is that the treatment processes are slow and require large pond volumes or media surface area. Two other technologies have emerged as leading alternatives: membrane bioreactors (MBR) and moving-bed bioreactors (MBBR). A basic schematic of the former is outlined below.
MBR is essentially an enhanced activated sludge process, wherein beneficial microorganisms consume the food and nutrients that enter the main vessel via the mixing zone. A recycle stream helps bring active, well-established organisms to the inlet of the mixing zone. A major difference of MBR from conventional activated sludge is the use of ultrafilter membranes rather than a traditional clarifier to separate solids from the effluent. The microfiltration process produces a very clear stream, essentially free of suspended solids. One deficiency of this most basic MBR process is that ammonia in the stream is converted to nitrite/nitrate, but the nitrogen remains. Thus, nitrogen can still serve as a nutrient in the plant cooling water system. If necessary, this problem can be solved by expanding the MBR system to include anoxic reaction chambers to promote denitrification, where microorganisms can convert nitrite/nitrate to elemental nitrogen.
With MBBR, the main reaction vessel includes mobile plastic media that serve as sites for the beneficial microorganisms to attach and then consume the nutrients and food from the influent. Because the reaction vessel is agitated with mechanical stirrers, settling, flotation, or filtration must be external to the vessel.
ChemTreat representatives can help plant personnel evaluate chemical requirements for the processes outlined above and recommend programs for the cooling and process systems fed with the resultant makeup water. Future blog posts will discuss methods to treat wastewater streams generated within the plant.
Please contact ChemTreat for assistance in designing a treatment program customized for your application. Like all other technologies, due diligence is necessary to determine the feasibility for utilizing methods. Always consult your equipment manuals and guides.