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Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>

4.3.2 Large scale technologies

Stormwater

Modeling

In large scale stormwater collection systems, computerized flow models are often used to evaluate control measures. Models can be proprietary software, calibrated to a particular system, or developed in house either fully or from combinations of proprietary software. Use of the models has demonstrated in some locations that less costly control measures can be used to satisfy local regulatory authorities effluent discharge requirements, particularly from combined sewer overflows.

Flow control

Large cisterns have been constructed in some major cities to hold stormwater for release after a storm event has passed, thus reducing the size of the peak flow on the system. They are often located under parks, for ease of construction, to avoid other underground utilities. For example, the Cole Park detention vault in Dallas Texas, can accept and discharge a 50 year rainstorm within 24 hours. It consists of thirteen parallel tunnels. The upper 33 percent of the vault drains by gravity, and the remainder is removed by a submersible pump station. The system protects a highway crossing whose 4 m diameter culvert could not handle the peak runoff rate.

Vortex valves are used in some locations to restrict flow to the sewer at higher flow rates, thus maximizing upstream storage in the system. The vortex induced by high flows causes an air core in the flow which reduces pipe discharge from a manhole.

Treatment

Simple floatables control using stainless steel underflow baffles installed in existing combined sewer bypass manholes will be installed in the Massachusetts Water Resources Authorities sewer system. Laboratory testing and computer modelling indicates the system should work well. Each baffle was designed separately for the flow conditions and dimensions of the existing structures.

Vortex separation is used to remove coarse settleable solids and floatable material, as well as grease, from stormwater flows and combined sewer overflows. The units are available in sizes ranging from manhole inserts to separate underground concrete vaults for higher flows.

Figure 4.6 illustrates one such system manufactured by Vortechnics Inc [http://www.vortechnics.com/]. Flow is introduced tangentially to the chamber, grit impinges on the walls and falls to the bottom while clarified flow exits from the top of the chamber. Oil and floating solids are trapped behind underflow baffles, or by submergence of the outlet pipe at higher flows. The systems are designed to prevent re-suspension and release of solids at high flows.


Figure 4.6: Vortechnics Inc. Stormwater Treatment System

Installations of Vortex separators include the largest Storm King installation is at the Columbus, Ohio Uptown Park Water Reclamation centre. A Vortex installation in Seabrook, New Hampshire discharges to a salt water marsh, and protects clam beds in a nearby harbour. Stormceptor systems, comprised of a deep manhole with a baffle insert which induces vortex flow and separates the manhole into a settling zone and a bypass overflow zone, have been installed throughout the U.S. and Canada [http://www.csrstormceptor.com/ and http://www.wateronline.com/].

A proprietary system, Stormtreat, combines solids removal with wetland treatment [http://www.stormtreat.com]. Inlet catch basins are fitted with grit filter bags to trap larger floatables, then flow is directed to sedimentation chambers fitted with skimmers, then to a gravel filter feeding a constructed wetland. From the wetland, flow enters a gravel bed which acts as a reservoir during higher flow, and allows infiltration into the subsoil.

In Houston, Texas, three facilities consisting of chlorination tanks using sodium hypochlorite for odour control, circular clarifiers for storage, solids and floatables removal, and outlet bar screens, followed by a natural bayou, have been installed for combined sewer overflow treatment. Clarifier underflow is either pumped or gravity drained to the sanitary sewer. The facilities were designed for a 2 year storm.

Wastewater

The 1996 US EPA needs survey stated that 190 million people, 73% of the US population, are served by 16,024 wastewater treatment facilities. Seventy one (71) % of the facilities are in small communities with populations less than 10,0000. Only 1% provide less than secondary treatment. (WET Newswatch Feb. 1998).

The 1996 Canadian Municipal Use Database states that 72% of the Canadian population is served by central wastewater treatment. (Note the survey does not contact 15% of the population in centres with less than 1000 pop.). Of those surveyed centres with treatment, 23.8% have primary treatment, 32.2% have secondary treatment, and 44% have tertiary treatment.

Preliminary treatment

Design philosophy now focuses on the removal of rags and floatables from wastewater rather than grinding, since screenings tend to be relatively non-biodegradeable and persist through the process. Therefore, comminutors are not usually specified anymore, especially in larger plants.

Mechanically cleaned coarse screens (greater than 6 mm openings) of varying types are used, with reciprocating rake types presently most popular. The trend is towards maximum screen openings of 13 mm to prevent plastics passing sideways through the bars. A trash rack may be placed upstream of the mechanical screens if the system is fed by a combined sewer. Manually cleaned screens are not generally specified except as protection in unused bypass channels.

Screenings are generally dewatered before disposal to reduce hauling costs. Various types of auger and press type dewatering machines are available.

Grit removal is standard practice, with aerated grit chambers used frequently. Grit removal from the chamber is effected by chain and bucket collectors, screw augers, clamshell buckets, recessed impeller pumps, or air lift pumps. Square unaerated detritus tanks are also used, complete with bottom rakes and grit augers. Vortex grit removal systems are becoming more popular because of their efficiency at removing finer grits at reasonable cost (WEF Manual No. 8). Mechanically induced vortex systems have been installed in large scale. Hydrocyclone systems (Teacup) which rely on head loss to generate a vortex are available up to 7.6 MLD for individual systems.

Captured grit is generally washed and dewatered before hauling to landfill for disposal. Methods include hydrocylones and various types of dewatering conveyors including paddle and cleated belt. Cleated belt dewatering conveyors tend to capture more and finer grit than the paddle types.

Equalization

Flow equalization is not generally considered economical for new plants treating flow from separate sanitary sewers with little infiltration and inflow and little industrial waste, but it is a useful tool for upgrading of existing facilities where the upstream sewer system has insufficient capacity to act as a buffer. Stored flows have usually undergone preliminary treatment to prevent buildup in the equalization tank, and the tank itself will be mixed and aerated.

Primary treatment

Sedimentation is the most common form of primary treatment. Circular tanks are currently most popular due to lower capital cost per unit surface area, followed by rectangular tanks. Square tanks are rarely used due to the difficulty of removing sludge from the corners. Stacked tanks are used where space is a problem, as at the Deer Island treatment facility in Boston, Massachusetts, and in Mamaroneck, New York.

Submerged launders are gaining popularity as an odour control measure. Wind protection is an issue in some areas, as in Milford, Iowa, to prevent surface currents. Scum collection equipment is heated in extremely cold areas to prevent ice formation. Two types of scum collection equipment are typically used, tilting trough and sloping beach. In warmer climates, sludge is withdrawn at higher rates in more dilute form to prevent septicity and solubilization, which can affect plant effluent quality.

Chain and flight sludge collection equipment is now constructed of plastic and fibreglass. Plow type collectors are used in circular tanks, and suction types are avoided on primary sludge due to clogging. Travelling bridge collectors on rectangular tanks cannot be used if the tank must be covered for odour control.

Preaeration enhances primary settling by promoting flocculation. Aerated grit chambers can perform this function. A Water Environment Federation survey of over 1000 treatment plants found that 29 percent use preaeration (Water Environment Federation; see 4.10 Sources of information). Some plants use chemical addition to enhance primary sedimentation. Iron salts are used in several cities in Michigan and at California's Hyperion plant, aluminum salts in Maysville, Ohio and Milan, Michigan, and lime at Martinez, California. Lime is not used often due to higher sludge production and chemical handling difficulties. Chemical coagulation allows operation of tanks with higher overflow rates, thus requiring smaller amounts of tankage.

Thickening in the primary clarifier is generally only practiced at lower overflow rates to prevent scouring of the sludge blanket at higher flows. Co-thickening with Waste Activated Sludge (WAS) in the primary clarifiers is practiced, again at low overflow rates, in moderate climates.
Fine screens are sometimes used in lieu of primary treatment, but removal efficiencies are lower. Of over 1000 treatment plants responding to 1989 WPCF survey, 6 percent used fine screens. Types of screens used include inclined self cleaning static, rotary drum and rotary disk. Hot water sprays are required for cleaning of grease buildup, especially in colder climates.

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