Municipal Wastewater Treatment
1. Public demands cleaner water
Background
- Today, more than any other time in history there are ever increasing environmental concerns regarding water pollution in general and water scarcity as it relates to sources of “usable” water. While these topics are connected to water, the focus here will be centered on the water pollution and its impact on amounts of water suitable for typical uses such as public consumption, agricultural irrigation, industrial demands etc. With that said, water resources are one of the top environmental concerns facing mankind today. When considering demands being placed on the collective water supply, both water and wastewater, the source and level of treatment should be based on the ultimate end user requirements.
Regulatory History
- Water pollution had become more of a public issue since shortly after WWII marked by the Federal Water Pollution Control Act (FWPCA) of 1948. The 1950’s saw uncontrolled growth of industries with discharges containing a variety of pollutants, including chemical residuals, resulting in a corresponding amount of pollution that had gone largely unchecked. The FWPCA became the basis for a later action by the newly founded Environmental Protection Agency (EPA) that resulted in broadening the FWPCA in 1972 into what is known today the Clean Water Act (CWA). At that time, drinking water supplies for much of the country were more plentiful and demand based concerns for the same were considered a minor concern when compared to today. So other than widespread settlement of the American Southwest and other dry areas of the United States, which has had an ever growing demand for water, the major concern was preventing surface waters from being polluted to the point they were polluted beyond their natural state. With the advent of the CWA, came the need to regulate wastewater discharges to adequately prevent further pollution of the nation’s waterways as well as remediation of existing polluted waters which led to the National Pollution Discharge Elimination System (NPDES) permits in 1972. These permits, allowed the EPA to regulate and help eliminate source point polluted discharges that could lead to impairment of the waters of the United States. In most cases, the states had been given primacy regarding administering and regulating their own NPDES permit programs with the requirement that their permitted water quality standards be at least as restrictive as that of the EPA.
2. Regulations becoming more restrictive
- Similar to the changes in the regulatory environment that occurred between the end of WWII and the institution of the Clean Water Act, we are now seeing new NPDES permit requirements that are much restrictive when compared to those at the onset of this program. Presently, the general public is becoming more and more aware of the value of clean water making it more of a prized resource. This shift towards a greater appreciation for clean water has is not based solely on consumption, included also is the effects to the ecosystem at large as well as well as agriculture and a large number of other uses.
3. Improved operations, sometimes with chemicals
- Many of the treatment systems whose designs and expected treatment capabilities predated implementation of the more recent NPDES water quality standards are not equipped to provide treatment standards demanded of systems today. Today a lot of attention is being given to nutrient removal from waste streams prior to surface discharges. This nutrient group includes, but is not limited to; nitrogen, phosphorous and carbonaceous biochemical oxygen demand - 5 day (CBOD5). With that being said, the three primary options are treatment system upgrades, improved operational strategies and/or chemical programs. The first option requires significant capital expenditures, permitting, construction and transition. Most treatment system owners/permittees are not able to fund these types of projects without the ability to raise capital by either increasing utility rates and/or finding other sources of funding such as grants or loans. The second and more attractive option, improved operations, requires greater operational knowledge that often times does not include more conventional thinking. In this case, modification(s) to process control parameters may be enough to meet the new discharge standards. Zones designed to create more ideal conditions capable of removing individual pollutants can be created thereby increasing removal efficiencies. Earlier system designs incorporated a lot of overlap with minimal consideration to where individual sub-processes stopped and started. One of the more recent demands placed on treatment system operators is the need to control total nitrogen discharged from the plant. While many treatment systems can meet ammonia discharge limits, meeting nitrate limits is another story. Conditions necessary for nitrate removal (nitrates, organic source and detention time) are much different than that needed for nitrification. In consideration of that fact, one of the best locations in the plant able to provide more ideal de-nitrification conditions is at the “head” of the plant where the return activated sludge (RAS) and influent waste stream enter into the treatment system. It is at this point that the bacteria can use both combined oxygen from the nitrate laden RAS for respiration as well as organics from the influent CBOD5 for food. This will accomplish 3 benefits reduction of; nitrate concentrations, organics from the waste and electrical power needed to operate the treatment system. While this treatment could otherwise occur without process changes, the degree of effectiveness would be negligible.