The basic kinds of wastewater treatment processes are physical, biological, and chemical. Physical processes remove solids by such means as screening, skimming and settling. In biological processes, bacteria and other organisms are used to consume organic matter. Chemical processes can be used to act on pollutants in ways that allow them to be more easily removed from wastewater.
from the U.S. Environmental Protection Agency (EPA) explains the three conventional steps in wastewater treatment:
The conventional three-stage wastewater treatment process can be negatively affected by toxic chemicals in the waste stream. Such chemicals can disrupt the functioning of standard wastewater processes or can end up being harmfully discharged into the environment.
, for example, that "chromium can inhibit reproduction of aerobic digestion microorganisms, thereby disrupting sludge treatment and producing sludge that must be disposed of with special treatment." If they find their way into wastewater, volatile organic compounds (VOCs) can cause gases or vapors to build up in sewer head spaces, sometimes resulting in explosions. Chemical reactions in wastewater can cause poisonous gases to form. Cyanide and acid, says EPA, "both present in many electroplating operations, react to form highly toxic hydrogen cyanide gas." Similarly, "sulfides from leather tanning can combine with acid to form hydrogen sulfide."
Wastewater treatment at a paper mill. Credit: EPA[/caption
For these reasons, industrial facilities often have to use pretreatment processes to remove such compounds from wastewaters prior to conventional treatment. Pretreatment in the U.S. is a highly-controlled process falling under the 1972 Clean Water Act (CWA). EPA, in partnership with state governments and publicly-owned treatment works (POTWs) have the responsibility to regulate a National Pollutant Discharge Elimination System (NPDES), which issues permits for industrial firms that emit wastewaters.
Background materials from Munich, Germany-based Siemens AG
stress that industrial wastewaters are highly-specific to the facility. Impurities could include "acidic [chemicals from a plating process, colorings, acids, oils and fats from food processing, or the presence of organic chemicals, such as pesticides, paints, dyes, or detergents." Pretreatment processes, says Siemens, "can be as simple as chemical addition or as complex as the integration of multiple unit processes for a complete water treatment system." Pretreatment equipment can be purchased directly or operated on-site through a service contract.
Siemens cites the case of a utilities company in Texas
that had a problem with copper levels substantially higher than those allowed under its NPDES permit. Siemens provided an ion exchange system for removal of heavy metals. The system uses 30-cubic-foot vessels containing cation resin to remove copper from the facility's wastewater. Siemens is contracted to remove the vessels periodically and take them to a special facility for recovery of the copper and regeneration of the resin. Siemens says that "all residuals from the regeneration process are sent for secondary treatment and recovery" and that "no waste goes to a landfill." Siemens then ships the regenerated resin back to the utility for reuse.
Tertiary Treatment Provides Final Polishing
As I mentioned above, tertiary treatment really refers to any final advanced treatment processes that prepare wastewater for discharge into the receiving environment, such as a river, lake, wetland, or the ground. Such processes might include further filtration, lagooning, land treatment, or removal of nutrients or other substances. According to Siemens, tertiary treatment technologies "can be extensions of conventional secondary biological treatment to further stabilize oxygen-demanding substances in the wastewater, or to remove nitrogen and phosphorus." Such advanced treatment can also involve "physical-chemical separation techniques such as carbon adsorption, flocculation/precipitation, membranes for advanced filtration, ion exchange, dechlorination, and reverse osmosis."
Dow chemical plant in Freeport, Texas. Credit: Dow Chemical Co.[/caption
Under appropriate circumstances, land treatment can be a beneficial, lower-cost tertiary alternative. EPA refers to land treatment
as "the controlled application of wastewater to the soil where physical, chemical, and biological processes treat the wastewater as it passes across or through the soil." The most common land-treatment technique is slow rate infiltration, using the treated wastewater for irrigation. Most nutrients are used by plants, while "other pollutants are transferred to the soil by adsorption, where many are mineralized or broken down over time by microbial action."
Constructed wetlands are another tertiary strategy. My recent article on green infrastructure
detailed three cases of engineered wetlands constructed by The Dow Chemical Co., Alcoa and Shell Petroleum Co. Shell built a constructed wetland at one of its oil fields in Oman, where its wells were bringing up large volumes of water along with the oil. The wetland uses reed beds to filter the water. Microbes break down the oil underground. Not only can such green infrastructure projects provide cost-effective tertiary treatment, but they also create wildlife habitat while eliminating harmful pollutants.
I talked about Dow's wetlands projects with Gena Leathers, who manages the company's corporate water strategy. Dow's subsidiary Union Carbide built a 110-acre wetland at a fraction of the cost of a conventional gray-infrastructure treatment system. Leathers told me that the purpose of the wetland "was to provide a finishing step to reduce solids in the effluent to meet permit requirements. This system, she said, "saved the company many millions of dollars while providing value to nature." She stressed that "there are many things companies can do to help the company and nature at the same time."