Wastewater Collection

Conventional wastewater collection systems are an ancient technology probably originating at least 3000 years ago in Crete or Babylonia. During the height of Roman civilization almost all of the major cities had sewers (cloacae) in essentially the same version as we use in modern cities. Although conventional sewers apparently work well there are some serious problems that must be overcome in design, construction and maintenance.

Conventional sewers require strict alignments in both the horizontal and vertical plane. Manholes are installed whenever direction and grade change, and to allow for cleaning. The minimum recommended slope is 2% which means that in cities without hills or natural gradients, 1000 feet of sewer will drop 20 feet, hence the requirement for complicated pumping stations.

Because sewers must carry both liquid and solids, maintenance costs of sewer systems often exceed the cost of wastewater treatment. Sewers generate odors and corrosive gases which form acids that slowly dissolve the piping and manholes. Eventually the sewers collapse creating the necessity for expensive repairs and/or replacement.

To solve the problems associated with conventional sewer systems a professor of civil engineering at Harvard, Gordon Fair, proposed small diameter collection systems using both gravity and/or small pumps. Because of the ease with which small diameter collection systems can be installed, they are significantly less expensive than conventional systems (see graph). As a general rule, small diameter collection systems can be installed for 40-60% of conventional systems. Unlike conventional systems, maintenance costs are 5-10% of the costs of conventional sewers, primarily because only liquid is transported.

Separation of solids from liquids is accomplished by placing an interceptor tank on the homeowner’s lot or using the existing septic tank. Because of the proximity to the home, water temperature in the tank is maintained and consequently, the digestion of solids is much more efficient than transporting via sewers to a treatment facility. In studies funded by the US EPA, the frequency of pumping to remove accumulated solids is on average 15 years.

Small diameter systems do not require manholes. Hence, gases are kept dissolved in the wastewater and cannot be vented into the air via manholes. Materials for piping are acid resistant and will not corrode. Lines can be set in the road with trenching equipment at the rate of up to 2000 feet a day. Shoring of trenches is not necessary because the small diameter sewer line is generally set at 30 to 60 inches below grade. As the figure shows, small diameter lines can go up and down hills.

Since 2/3 to 3/4 of the total costs of providing wastewater collection and treatment are in the costs of the collection system, significant capital savings can be realized with the adoption of small diameter sewer systems. Small diameter systems remove the constraints imposed on development by gravity and conventional sewer system designs. Sewers no longer need to go downhill; lot layout can be made independently of sewer lines.

Wastewater Treatment

If you thought a minute about the requirements for a wastewater treatment system for your community you might come up with the following list:

Effectively treat wastewater to the highest standard

Inexpensive to build and operate.

Produce no odors.

Be attractive.

Provide habitat for wildlife.

The last two criteria may seem like too much to expect. However, there is, indeed, such a technology that works extremely well with the small diameter collection system. It is part of a multi-step treatment system and is referred to as a Constructed Wetlands (see About Constructed Wetlands). Because Constructed Wetlands utilize the applied ecology of the wetlands or marsh, this wastewater treatment system has all the advantages of a natural system, that is self-maintaining, self-organizing and self-regulating. Consequently, Constructed Wetlands generally require no energy inputs and minimal maintenance. These factors make the Constructed Wetland an excellent solution for onsite wastewater treatment systems.

The Constructed Wetland is one part of a treatment process with multiple steps. The primary treatment (BOD and TSS reduction) occurs in an interceptor tank or a septic tank. Secondary treatment (BOD and TSS reduction, Nitrification and Denitrification, Pathogen Removal) occurs in a Constructed Wetland. Tertiary Treatment (BOD and TSS reduction, Nitrification and Denitrification, Pathogen Removal) often occurs in a recirculating sand filter. Additional treatment can be added to the process if the effluent will be used for irrigation or on-site reuse. See the diagrams below:

With the three treatment steps explained above the overall removal rate is 98%, when each element operates at the lowest levels of performance typically found in these systems. Other considerations are the multiple microbial ecologies. Multiple, distinct microbial ecologies provide different opportunities for microbial degradation of pollutants. Along with the multiple treatment steps and longer process time (21 times longer than mechanical treatment systems), there is significant reduction in endocrine disrupters, antibiotics, and personal care pharmaceutical products (see WE Research Journal). In summary, multi-step treatment is both reliable and effective in the wastewater treatment process.

According to NOWRA (The National Onsite Wastewater Recycling Association), Onsite systems provide wastewater treatment for many sectors including residential, commercial and industrial. Onsite systems now provide more than 40% of the wastewater treatment services to residential areas, communities, shopping centers and commercial businesses throughout the United States. On-site treatment supports the municipal infrastructure and can often provide solutions in difficult or remote areas. “On-site systems are an effective solution to protecting water quality. They are valuable component to integrate with watershed management plans and implementing sustainable development concepts.” (Source NOWRA -The National Onsite Wastewater Recycling Association)

The technology used for the collection and on-site treatment of wastewater has rapidly evolved in the last forty years. The design challenges associated with decentralized treatment systems have been addressed by selecting technologies that meet the following engineering design criteria:
1. Simple to construct
2. Simple to operate
3. Simple to maintain
4. Minimal energy requirements

The graphs below show capital costs and operating costs of several types of wastewater treatment systems. The data set is generated using an 800,000 gallon per day flow (roughly 13,000 people using approximately 60 gallons per person per day).

The data indicates the substantial savings that a small community of approximately 13,000 people will realize. The reasons for this are simple:

1. Only minimal energy is required for treatment.
2. Maintenance requirements are simplified.

These two factors are the most significant cost factors for conventional municipal wastewater treatment facilities. The added advantage of Constructed Wetlands is that the Wastewater treatment system can become part of the landscape, providing a significant aesthetic element and site featureas well as a wildlife habitat.

Regulatory Status and Design Criteria

The technologies discussed above are non-proprietary, public domain technologies. The University of Wisconsin and the American Society of Civil Engineers jointly offer courses throughout the United States. The EPA and the Water Environment Federation have design manuals to assist both the engineer and regulatory agency in designing and permitting these technologies.