Wastewater Treatment Technology
Virtually every paper looking at the future of wastewater reuse mentions the advances in membrane technology as a major part of its evolution. In fact, the Orange County Water District’s new Groundwater Replenishment System will implement a system using microfiltration and reverse osmosis membranes to create near distilled water from secondary wastewater effluent, and therefore abandoning the use of activated carbon filters. There are several different types of membranes, and the technology is classed by membrane pour size.
Membranes are generally used as an advanced treatment method, removing the smallest particles in the water, including bacteria and viruses. Reverse osmosis membranes can also be used for desalination. These processes use a lot of power, as water must be forced under pressure (up to 600 psi for RO membranes) into the membrane (Hammer, 2004).
Membrane fouling is the most common problem associated with membranes. As the membrane filters particles, they build up along the surface and eventually will clog the pores. While back pulsing slows fouling, other operations must be completed to keep the surface of the membrane clean and avoid fouling. Water that is filtered using membranes must be relatively low in suspended solids, as excessively large particles can damage the membranes and cause the membranes to foul.
Activated carbon systems are currently used mainly in the beverage industry to further purify product water, but are not being used in municipal wastewater treatment because of their cost. Carbon adsorption basically involve large carbon beds that have been prepared by controlled combustion to develop adsorptive characteristics of the carbon surfaces (Hammer, 2004). Hydrophobic organic compounds are removed by the process of adsorption. These processes take advantage of the accumulation of solutes at liquid-solid interfaces (Tchobanoglous, 1987). When water is sent through the activated carbon system, the contaminants dissolved into the water will be adsorbed into the carbon surface - they accumulate on the walls. The carbon is then removed from the bed, regenerated (cleaned) and then reused. A diagram of the process can be viewed at http://www.frtr.gov/matrix2/section4/94p-3314.gif .
Primary treatment is generally the first step in the wastewater treatment process and is designed to remove the largest contaminants in the water. These solids are likely to clog pipes and damage equipment if not removed before the other processes.
Mechanical screens have opening of ½ to 1 inch, and mechanical rakes lift the solids from the water, where they are eventually taken to the landfill (Hammer, 2004).
Shredders are used to ensure that larger, flexible materials that pass through the screens are chopped down so that they do not damage equipment further down the line.
Grit chambers are designed to remove particles with 0.2 mm diameter; things such as sand, coffee grounds and seeds. There are different types of grit chambers that can be used, for these purposes, including clarifiers and forced vortex units.
Disinfection processes are used to inactivate pathogens, including bacteria and viruses. They are non-toxic to humans and are relatively cheap. Chlorination, ozone, and UV treatment are the most widely used methods of disinfection.
UV rays created by the bulbs shown below are absorbed into the DNA of microorganisms, preventing them from reproducing. The light needs to be in a very specific frequency - from 250 - 256 nm. The most efficient lights are low-pressure mercury. Coliform bacteria can be limited to 100 to 200 per 100 mL using UV treatment, although no residual is provided. (Hammer, 2004) UV is currently the fastest rising form of disinfection, being looked at more closely by many in the water reuse field because of the absence of chemicals needed to be added to the water in this process. The Groundwater Replenishment System (formerly Water Factory 21), considered by many to show the future of water treatment, is currently installing UV with hydrogren peroxide as it's disinfection process before replenishing Orange County's seawater barrier.
Trojan Technologies - Trojan UV 3000B
While hydrogen peroxide is currently not used very often in treatment systems, but the field is growing. This chemical has been found to be good for H2S control, as well as a second part of the disinfection (UV followed by hydrogen peroxide). Hydrogen peroxide will be used at the Groundwater Replenishment System in Orange County along with UV to disinfect the water before it is sent into the ground. For more information on Hydrogen Peroxide, please see the US Peroxide website.
Chlorination involves the addition of chlorine gas or liquid into the water, which is initially rapidly mixed. The water is then sent through a plug flow basin for approximately 30 minutes to allow for appropriate contact time with the pathogens. Bacteria and viruses generally need to be in contact with the chlorine for specific lengths of time (depending on the bacteria or virus) in order to be killed. Chlorine is the most widely used method of disinfection in the United States (Tchobanoglous, 1987). The main problem with chlorine is the production of trihalomethanes as by-products of the disinfection process. For this reason, the water treatment industry is looking at other methods of treatment as a replacement for chlorine. It is likely that chlorine use will eventually be completely abandoned.
Ozonation is an oxidant that rapidly disinfects microorganisms. It is used to oxidize iron, manganese, sulfide, nitrite, organics, pesticides, VOCs. While ozone works using short contact times, it does not provide a residual. This means that it is possible for bacteria to grow within the distribution system. Chlorine is generally added after the ozonation to prevent this from occurring (Hammer, 2004) Although ozonation has been mainly used in Europe, it is currently gaining support in North America. (Zytner, 2003) There are several ozone contacting processes available, and one of them is shown below:
From: Magara et. al, 1995
Hammer, Mark J., Hammer Jr., Mark J. (2004) Water and Wastewater Technology. New Jersey: Pearson Prentice Hall.
Magara, Y., Itoh, M., Morioka, T. (1995) Application of Ozone to Water Treatment and Power Consumption of Ozone Generating Systems. Progress in Nuclear Energy. 29: 175-182.
Tchobanoglous, G., Schroeder, E. (1987) Water Quality. Don Mills: Addison Wesley
Zytner, R.G., (2003) Water Quality Class Notes. Guelph: University of Guelph School of Engineering.