Key Design Considerations, Recent Ruling
Wastewater, stormwater collection systems
Municipalities face an increasing need for repair and rehabilitation of existing wastewater and stormwater systems. This need has arisen as a result of neglect, deterioration, or inadequate hydraulics resulting from development. Pipes are the key element comprising a sewer system - along with related pipe restraints, couplings and other fittings. The pipe system conveys the stormwater runoff to a selected body of water such as a river, and/or wastewater to a treatment facility.
Cylindrical concrete pipe is the most commonly specified for storm, sanitary and combined applications in larger diameters up to 144 in. But vitrified clay, with its good corrosion resistance, is still a popular choice for sanitary sewer projects with pipe sizes not exceeding 42 in.
What determines the size and construction material of the collection system pipes? Local conditions such as rainfall, temperature, soil characteristics, above-ground traffic loads, number of tree roots, slope (and other hydraulic considerations), and the abrasive/corrosive properties of the wastewater.
The following are some of the techniques that can be used, other than excavation and replacement, to rehabilitate the different components of wastewater collection systems:
- cured-in-place inversion linings - especially effective in high-traffic conditions or areas with building congestion;
- microtunneling with subsequent insertion of some type of pipe material;
- fold and form liners;
- pipe bursting systems which make way for a pipe to be inserted;
- chemical grouting methods;
- systems for repairing manholes and re-establishing lateral connections; and
- plastic liners and coatings.
However, extreme conditions calling for excavation and replacement include inadequate capacity and crushed, collapsed, or grossly misaligned pipes.
Since one of the primary design objectives in planning new or repairing old combined sewer systems is avoiding overflows of stormwater contaminated with sewage, temporary storage capability can be built into the facilities. Installing flow control devices is an effective way to do this, and are often combined with relief sewers and catchment basins.
Flow regulators can be installed at strategic points in the collection network to retard the flow during a storm and utilize the storage capacity of the pipes.
Integral to any wastewater collection system are pumping stations. These can be remotely controlled using supervisory control and data acquisition (SCADA) systems and other advanced instrumentation.
As the use of this technology continues to expand, pumping stations are increasingly being linked to entire area networks. Central automatic control is asserted from the treatment plant. As a primary information source, automatic sampling at various locations throughout the system can be applied.
Recent Stormwater Ruling
The EPA issued a final ruling on the Clean Water Act (CWA) Phase II stormwater regulations on August 7, 1995 - 10 months after the original October 1, 1994 deadline. As a result of the ruling, small municipalities, industries, and contractors will now be protected against suits by citizens.
The roots of this ruling go back more than two decades, when Congress established the National Pollutant Discharge Elimination System (NPDES). It resulted from a 1972 amendment to the Federal Water Pollution Control Act (which later became the CWA) to make point source discharges of pollutants into U.S. waters unlawful unless authorized by a permit. When the 1987 amendments were passed, stormwater discharges were divided into two phases, exempting the smaller "Phase II" discharges from regulation until October 1, 1994.
Some Phase I large stormwater discharges were not covered by the NPDES permit moratorium. Rules that were published in 1990 and 1992 required the large discharges to have individual permits. Discharges included in these rulings were from municipal separate stormwater sewer systems serving 100,000 or more citizens, industry, and construction projects affecting more than 2 ha (5 ac).
The Phase II program was to cover municipalities with separate stormwater sewer systems serving 100,000 or fewer citizens, small industrial activity, and construction projects affecting less than 2 ha of land.
At the insistence of environmental groups and dischargers, the EPA finally established a two-tiered approach for Phase II regulation in its August 7, 1995 rule. Now first-tier sources who receive notice must apply for a discharge permit within 180 days. For those not defined as Phase I or Phase II first-tier dischargers, the deadline for applying for a permit may be as much as six years away.
The EPA's Stormwater Phase II Subcommittee held its first meeting in September to recommend strategies for controlling stormwater discharges. Fairness, cost-effectiveness, and flexibility were stressed as the key goals of the program. They also discussed several obstacles to policy making: variable sources of runoff, regional differences in stormwater flows, and lack of research on controlling stormwater.
Looking ahead, the subcommittee plans to present its final report by December, 1996 for the September, 1997 stormwater regulation.