Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>
4.3.2 Large scale technologies
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.
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.
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
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
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.
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.
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.
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.
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.
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
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.