In the United States there are estimated to be 600,000 miles of wastewater sewer lines; networks of pipes, pumping stations, and other equipment that move sewage from toilet and sink to wastewater treatment plants. Many of the oldest sewer systems dating from before the turn of the 20th century are still in service in our largest and oldest cities, and much of the sewer piping in the United States is original. As cities grew, the need for sewage and stormwater removal became necessary to protect human health. The oldest sewer systems were designed to carry both the stormwater and sanitary waste together in one system (to save money the sanitary sewers tapped into existing drain, storm and canal systems) to the nearest natural water body. Our rivers and bays had a limited capacity for dilution and as populations grew were overwhelmed by the sewage and became open cesspools of vermin, filth and foul orders devoid of all aquatic life. To alleviate the health hazards and disgusting pollution we began treating sewage waste. Until the 1960’s many sewage treatment plants only used screens and large settling tanks to remove solids and debris from sewage before releasing the effluent.
Today those steps are called primary treatment and all sewage treatment in the United States includes additional steps to ensure public health. Secondary treatments usually include biological and/or chemical treatment. One of the most common biological treatments is the activated sludge process; in which primary wastewater is mixed with bacteria that break down organic matter and cleans the water. Oxygen is pumped into the mixture. A clarifying tank allows sludge to settle to the bottom and then the treated wastewater moves on for tertiary treatment at advanced wastewater treatment plants. Coagulation, filtration and disinfection take place in tertiary treatment which also serves as a barrier to viruses, captures organics leaving secondary treatment, and precipitates heavy metals and other suspended particles.
The existing wastewater treatment systems in our cities cannot process the combined flow of stormwater and sewage and our cities struggle with solutions that they can afford. Some, like San Francisco have built a system of storage/transport boxes. The storage/transport boxes are huge underground rectangular tanks or tunnels that surround the City, the SFPUC describes them as a moat, a fitting image as the storage boxed ring the city. The storage/transport boxes catch the combined stormwater and sewage as it overflows the sewer system, but before it reaches the shoreline of the Bay or Pacific Ocean and hold the water until it can be processed. The storage/transport boxes in San Francisco have a total storage capacity of 200 million gallons and hold stormwater and sewage for later treatment at one of the wastewater treatment plants.
In Washington DC, the Blue Plains sewage treatment plant is currently engaged in a $7.8 billion 20 year improvement program -the Clean Rivers Project adding a new stormwater storage tunnel that will hold 31 million gallons to the existing system storage for a total of 157 million gallons spread over the Anacostia River tunnels system and the new Blue Plains Tunnel. This will allow flow from the sewer collection system that exceeds the treatment capacity of the plant to overflow to the tunnel and be dewatered through a new enhanced clarification facility with a capacity of 225 million gallons a day. Retention basins, tunnels or storage tanks that can hold sewage until the water volume has eased or reconfiguring or expanding treatment facilities to increase maximum flow rates are capital intensive projects. Some cities like New York and Philadelphia, have targeted reducing stormwater flow using “green infrastructure” and low impact development strategies (LID) with BMPs (best management practices) to increase infiltration of rain and reduce the volume and velocity of stormwater.
In an existing city it is extremely hard to implement and maintain enough LID strategies to eliminate all excess stormwater flow and in NewYork City they are attempting to use the sewer piping system itself for additional storage by installing inflatable dams to block the flow of rainwater and sewage into New York Harbor in Brooklyn. If the water pressure in the pipes gets too high, threatening to back up sewage into homes or onto streets, sensors are supposed to deflate the dam to release some water. It is a really interesting idea costing only $15.7 million for two inflatable dams, but runs the risk of increasing pressure on an aging sewer pipe infrastructure. No doubt this has all been considered and the inflatable dams are located in areas where sewer pipes have been replaced, relined or repaired. Effective storage volume increase will be 2 million gallons for each dam.
No piping system can last forever and without continual maintenance, replacement and upgrade we have increasing instances of sewer pipe and system failure. Failing sewer pipes can pose a significant threat to public health and the environment. Systems with inadequate hydraulic capacity and/or blockages in the sewer pipes (not from inflatable dams) may lead to sanitary sewer overflows and sewage backing up into homes or onto streets. Untreated sewage potentially contains pathogenic microorganisms such as viruses, bacteria, and protozoa. Pipe failures can be caused by hydraulic restrictions (e.g. blockages intentional or caused by debris and fats, oil and grease buildups), hydraulic capacity (the pipe being too small for the flow), and structural condition of the pipes (failure due to deterioration).
Our growing and shifting population requires investment for new sewage infrastructure and maintenance and upgrade of the existing sewage infrastructure. In addition, current sewage technologies and management approaches may not be adequate to address emerging contaminants and health threats and are certainly not adequate to maintain reliable and sanitary sewage service to the 70%-75% of the homes and businesses that are on public sewers. The U. S. EPAhas estimated that if spending for capital investment and operations andmaintenance remain at current levels throughout the country, there will be ashortfall of approximately $270 billion over twenty years for maintaining, replacing and upgrading our wastewater infrastructure, and no source of funding to make up the shortfall.
As Rose George author of the “Big Necessity” who has studied sanitation issues and practices around the world (and written a very interesting and engaging book), points out it is not a socially acceptable topic of conversation. She believes that may be one of many reasons why wastewater infrastructure is crumbling in the United States, despite its critical importance. Out of necessity (or eroding manners) the unspeakable is becoming more often spoken of. We can no longer ignore our sewage infrastructure. No infrastructure lasts forever and we have failed to properly maintain and plan for the orderly replacement of sewage collection and treatment systems. In the United States we have never experienced the need for pipe replacement on a large scale and have taken for granted what we were given. Now we need to find a way to maintain, improve and upgrade our sewer systems and wastewater treatment plants as they become essential components of our water supply systems because the United States has slowly and quietly begun to address the availability of water by recycling wastewater. Sewage is after all 99.9% water.