A New Public Work Challenge: Nutrient Pollution and Sewers.

By: Peter Maier, PhD,PE
March 17, 2015

Due to the excessive algae problems, causing dead zones and closures of drinking water plants, the focus of solving water pollution is now on nutrients (fertilizers), mainly blamed on the runoffs from farms and cities. The latter requires first a closer look at how waste in cities is collected and how it impacts open waters. This, in order to avoid making the same mistakes that caused the failure of the Clean Water Act, due to a lack of understanding how an essential water pollution test should be properly applied.

Cities worldwide have settled on the shores of rivers, lakes and oceans for many reasons, among them the convenience of getting rid of their waste. Waste was dumped on the streets when indoor plumbing did not exist, while human waste was collected by local farmers to fertilize their fields.

The first sewers built by cities were replacing open drains solely intended to collect and dispose waste and rainwater from streets into nearby open waters. Only later, when houses received indoor plumbing, their plumbing drains were connected to these storm sewers and such a system is now called a combined sewer system, earlier discharging its waste, without any treatment, directly into open waters.

When concerns were raised about sewage causing water pollution and cities were building sewage treatment plants, engineers considered rainwater clean and its treatment a waste of money. To avoid having to treat this ‘clean’ rainwater, newer cities and city expansions were designing separate sewer systems. One network of pipes, called sanitary sewers for sewage, directing sewage to a sewage treatment plant before it is discharged into open waters and another separate network of pipes and open drains, called storm sewers, to directs rainwater runoffs, directly into to local open waters without any treatment.

When the regulations required that all the sanitary sewage had to be treated, hence also that from combined sewer systems, engineers were able to modify such systems that most of this collected mixture of sewage and stormwater, would be directed to a sewage treatment plant, but that during large rainstorms, part of this mixture (overloading the system) would be released from overflow structures, called CSO (Combined Sewer Overflows). Environmentalists and the public consider these CSO’s a major cause of water pollution, since part of this overflowing mixture is raw sewage.

Because of the ‘rainwater is clean’ argument, most public works engineers still consider separate sewer systems better than combined sewer system. Most older cities have combined sewers systems and consider this raw sewage mixture from CSO’s highly objectionable, although the raw sewage in such overflows is very diluted. The public opinion now is forcing city officials to look for CSO solutions, thereby unfortunately still convinced (told) that their combined sewer collection system is obsolete and thus require replacement or modifications.

Most cities in The Netherlands are old and do have combined sewer systems, so there too, concerns with these combined sewer systems were raised in the sixties and seventies. Mainly because of the horrendous cost of converting these combined systems into separate systems, engineers evaluated the entire water pollution picture, thereby realizing (even without the nutrient impact) that stormwater is not clean and that the mixture of overflows from CSO’s is mostly water. After a complete waste load analysis (total waste loading on open water) of combined systems, depending their CSO overflow frequencies, the result was that combined sewers systems are actual better (less pollution loading) for the environment, than separate systems. Although separate systems are still designed, the Dutch government now is requiring engineers to design combined systems with limited overflow frequencies. Capacities of such systems are now expressed in how much more hydraulic capacity (wet weather flow or WWF) is required for their dry weather flow (DWF). Most cities now require that the WWF capacity is 5 times their DWF capacity. This, while regulations in Holland already in the seventies addressed, unlike in the US, the oxygen depletion pollution caused by nitrogenous (urine and protein) waste in sewage.

Nitrogenous waste (urine is mostly urea) first becomes ammonia, what is biochemically oxidized into nitrates (thus requires oxygen), but in all its forms is a fertilizer for algae growth. Nitrates can be reduced to nitrogen gas, but this requires a special biological process, called denitrification. Most people accept acid rain, but do not know what green rain is. This is rain containing reactive nitrogen (single nitrogen atom) ready to fertilize plants. On land this green rain stimulates the growth of grasses and brush, during wet seasons, what during the dry seasons becomes the kindle wood for hard to control range and wildfires. When this green rain runs off from fields or especially hard surfaces in cities, it also washes these hard surfaces (roofs, streets parking lots, yes also golf courses and lawns, already saturated with synthesized fertilizers) and this ‘supposedly clean’ stormwater in separated sewer systems will be discharged, without any treatment, into local open waters.

The nitrogenous waste (urine and protein) in sewage, is not required to be treated under the Clean Water Act, as the result of a faulty applied essential water pollution test. Too embarrassed having to admit that such a mistake was made, EPA insist that this, now called nutrient pollution, was the responsibility of individual states, while states in turn (probably for the same reason), is claiming it to be a federal responsibility. As with so many problem, unnecessary legal lawsuits (in this case brought by environmental interest groups) are filed and courts will have to decide who should be responsible. Thereby again wasting time and money to correct this essential test and force EPA to implement the CWA as was intended by Congress.

Summarizing the advantages and disadvantages of the two sewer collection systems in the US:

1. Separate sewer systems.
Advantage: No raw sewage overflow (CSO) and all sanitary sewage is treated(?)
Disadvantage: Very costly, because of two networks of sewer pipes. Stormwater is not treated and the nutrient pollution in sewage is also not required to treated. Providing nutrient treatment for stormwater and sanitary sewage, when required in the future, will be very expensive.

2. Combined sewer systems.
Advantages: Less costly, because they use only one network of sewer pipes. Most of the sanitary sewage and the first flush of stormwater is treated.
Disadvantage: raw sewage, although very diluted, is discharged from CSO’s only during rainstorms when the hydraulic capacity of the system is exceeded. The damage to the environment depends on the frequency of such overflow occurrences.

Since especially ‘nutrient’ pollution now is considered a major cause of water pollution, it is time to reevaluate all our present programs that deal with water pollution and especially evaluate our present theories how cities should collect and treat, not only their municipal sewage, but also their storm water. Increasingly important since most sewage treatment plants do not treat nitrogenous waste and that the rainwater runoff from cities is loaded with nutrients and other pollutants.

Most of cities with combined sewer systems are presently contemplating or already implementing solutions by either converting their systems or building large holding tanks for their CSO overflows. This, again at a horrendous cost to the public and without considering what the impacts of such solutions are on the water pollution of open waters. In addition, most will not realize that, when nutrient regulations will become required, this will require very costly modifications to their existing sewage treatment facilities, while such modification also often jeopardize the treatment reliabilities of such facilities.

Therefore, before any decisions are made, the public (city administrators) should first demand answers on the following questions from their public works departments in charge of sewage collection and treatment.

1. How is our own sewage presently treated, especially what happens to the nitrogenous (nutrient) waste? This treatment evaluation should be based on the proper use of the BOD test and all the necessary nitrogen test data.

2. What happens now to our stormwater, what is its composition and how does it impact our open waters?

3. How sensitive (limited) is our sewage treatment plant for large hydraulic load fluctuation, as the result of rains or with other words, what is the overflow frequency of the CSO’s?

Without such essential information, the changes are great that similar mistakes are made when the EPA implemented the CWA and by using an essential pollution test incorrect, failed to address nitrogenous waste. Thereby not only causing the failure of this second largest federally funded public works program, but also wasting billions of public funds. Hopefully that will not happen again.