also become accepted technology for raw wastewater treatment in smaller applications. (Buecker, 2013) Regarding MBBR, the main reaction vessel includes mobile plastic media that serves as sites for the beneficial microorganisms to attach and then consume the nutrients and food from the influent.
to inhibit corrosion. However, of growing concern is the discharge of phosphorus to natural bodies of water, and the effects such discharge has on proliferation of toxic algae blooms. Phosphate also drives algae growth in cooling towers, increasing chlorination costs and the production of undesirable chlorinated organics.
Figure 5 (Left). Algae completely covering a pond surface. and Figure 6 (Right). Algae hanging from plastic tower fill.
Figure 4. MBBR reaction vessel diagram showing mobile media.
MBBR can in some respects be thought of as a very advanced version of the trickling bed wastewater treatment process, where the beneficial microbes were attached to fixed media and then consumed nutrients and food as water flowed over and along the media. Due to the use of solid media in the reaction vessel, filtration membranes cannot be placed in this compartment. Rather, filtration must be performed separately.
At many locations now, phosphorus discharge is limited if not entirely banned. Also being restricted is discharge of metals, including zinc; a common key ingredient in phosphate/phosphonate formulations for additional corrosion inhibition. These restrictions have led to development of alternative, polymer-based programs that offer a more sustainable alternative to algae control in cooling systems. Polymer formulations containing the carboxylate group have been successfully utilized for decades to control calcium carbonate (CaCO3 ) scale in cooling water.
One concern often raised at industrial plants is providing staff, both from a quality and quantity standpoint, to operate these seemingly complex systems. Two ideas immediately come to mind. First, if the plant is located close to the POTW, it may be possible to place the MBR or MBBR at the POTW, and have that staff operate it. Secondly, many of the reputable manufacturers offer â€œbuild-ownoperate-maintain (BOOM)â€? programs, where, for an annual fee, they will take care of all equipment details. Even with good pretreatment of these or other makeup water streams, additional issues continue to gain momentum. One is increasing concern with phosphorus discharge to the environment, and how to reduce such discharge in cooling tower blowdown while still protecting the cooling systems from corrosion and scale formation.
Cutting-Edge Cooling Water Treatment
An evolution is underway regarding scale and corrosion control in industrial cooling-tower based systems. For four decades, the most common treatment programs have been based on a core chemistry of inorganic and organic phosphates (the latter typically go by the name of phosphonates or phosphinates) that combine with potential scale-forming elements, most notably calcium, and whose reaction products precipitate at anodes and cathodes of metal surfaces CTI Journal, Vol. 40, No. 2
Figure 7. Carboxylate functional group.
However, many other scaling compounds are possible, including calcium and magnesium silicates, calcium sulfate, calcium fluoride, and manganese dioxide, to name some of the most common. The need to combat these and other scale-formers has generated development of co- and ter-polymers, containing more than one functional group. The polymers inhibit scale formation by two mechanisms, crystal modification and ion sequestration. A low part-per-million residual is often sufficient to inhibit scale formation, but the choice of polymer or polymer blend is in large part dependent upon the chemistry of the cooling water. That is why, especially for new plants, comprehensive, and ideally historical, makeup water analyses are necessary. Too often, project designers only receive partial water quality data, which makes it very difficult to select proper treatment chemistry 53