1. Pre-Treatment
Pre-treatment is the
first device the sewage enters. Pre-treatment may be provided by using a
trash trap (holding tank), communitor (sewage grinder) or a bar screen
(sewage straining device). Any of these elements may be employed - they
are all designed to accomplish the same task which is to pre-treat or
condition the wastewater before it enters the aeration chamber of the
treatment plant.
Trash traps are holding tanks that are assigned to trap floating material and they have a space at the bottom so they will trap solid material before it enters the aeration tank. Normally they are sized to provide about 15 percent of the rated treatment plant capacity.
Communitors are sewage grinders that shred the solid material in the raw wastewater flow into small particles. They are equipped with an electric motor and hardened steel cutters that reduce the solids in size as they pass through the communitor.
The third type of pre-treatment device is a bar screen. It is made up of a series of bars arranged in parallel to trap large objects that may be in the wastewater flow. Bar screens are usually installed with a communitor to protect it from large untreatable or metal objects that might damage the cutters.
2. Aerated Flow Equalization
A precast concrete aerated flow equalization facility is installed to
provide temporary storage capacity for anticipated surges in the daily
wastewater flow. The facility is designed to protect the hydraulic
reliability of the secondary wastewater treatment system when the peak to
average diurnal flow ratio exceeds 4:1 or when a significant runoff period
of less than 18 hours per day is encountered. The aerated flow
equalization facility is designed to precede a wastewater treatment plant.
The holding capacity of the flow equalization device exceeds 8 hour
retention capability of the design daily wastewater flow.
The flow equalization facility is located downstream of preliminary treatment equipment such as pretreatment chambers, communitors or bar screens so that grit and other debris will be removed. The facility is located upstream of all secondary wastewater treatment plant tankage so that all raw wastewater flow shall pass into the equalization tank for controlled application to the aeration tanks.
3. Extended Aeration and Diffusers
It is necessary to continuously mix the contents of the aeration tank
with the incoming raw sewage to ensure that the activated sludge within
the system is brought into constant contact with the incoming sewage. The
amount of air being introduced and the distribution of the air along the
side wall of the tank will determine the degree of mixing within the
aeration chamber.
4. Surface Skimmer
A surface
skimming system is provided with each Norweco sewage treatment system.
They are provided to remove floating particles from the surface of the
final settling tank and return them to the aeration chamber of the
treatment plant.
5. Foam Spray System
Foam control
spray systems are provided on all larger installations and most of the
smaller ones, especially if the plant is expected to be under loaded or if
it is to receive a high detergent flow. Foam spray devices, like surface
skimmers, are a handy tool and may be used for routine plant maintenance
and elimination of a nuisance.
6. Clarification Tanks
Final settling tanks in the treatment plant are designed to remove
activated sludge and organic material from the contents of the plant. They
are, as their name indicates, settling chambers where the tank contents
are held about 4 hours to permit all solid material to settle to the
bottom of the chamber. The lower area of the tank is designed to form a
hopper. Side walls are slanted in the center of the tank to provide a
small area where the sludge will accumulate. Here the settled sludge is
picked up by the air-lift sludge return pump and pumped back to the
aeration chamber for final treatment.
7. Sludge Return
The air-lift
sludge return system is used to pump settled sludge out of the hopper in
the settling tank and back to the aeration tank to complete the treatment
process. Norweco sludge returns are capable of pumping in excess of 200
percent of the daily flow.
8. Sludge Holding Tanks
Sludge
holding tanks are provided with wastewater treatment plants. They are
installed to store excess sludge until it can be disposed of or returned
to the treatment plant during low sludge periods. Normally, sludge holding
tanks are sized to provide 10 percent of the rated treatment plant
capacity and are equipped with air diffusion equipment. As such, they are
called aerated sludge holding tanks and provide odorless sludge storage
digestion.
All Norweco systems with sludge holding tanks are designed to provide extra capacity in the treatment plant blowers so that the sludge holding tank may be aerated on a continuous basis without reducing the air supplied to the treatment facility. In addition, the holding tanks are equipped with overflows to permit the sludge to return to the aeration chamber of the treatment plant.
9. Cycle of Operation
Twenty-four
hour time clocks are used to cycle operation of the equipment and obtain
proper aeration rates. The plant clock has 96 individual tripper prongs
and each prong represents 15 minutes in the overall 24 hour period of
operation. The time cycle for plant operation is adjusted so that optimum
treatment may be obtained. Secondary or weekend time clocks may also be
employed to switch control over to another unit that will change the
aeration rate. The ultimate time cycle and aeration rate must be
determined by working closely with each individual system and adjusting it
to best serve its particular facility. Some systems may have changing
flows and not reach their ultimate cycle for months or, in some cases,
even years. It is important to observe and adjust the aeration rate as
required by the wastewater load.
10. Tertiary Treatment
Tertiary
treatment or "third stage" treatment is provided after primary and
secondary treatment of wastewater and may be accomplished in a number of
ways. Wastewater plant effluent may enter an effluent lagoon that is sized
to hold the liquid for a term. Tertiary treatment may also be accomplished
by using a sand filtration device. They are designed to filter treatment
system effluent through sand before discharging it to the receiving
stream. Micro strainers have also been used. They are mechanically
operated devices that automatically strain the final effluent. Generally
they must be installed in a heated building, and therefore, are limited to
use on large systems.
Of the above listed devices, the most commonly used are the smaller effluent lagoons and surface sand filters. Both are widely used and of the two, generally surface sand filters are preferred because of the land loss involved in construction of an effluent lagoon. Many elements come into play when selecting a tertiary treatment device - the size and condition of the receiving stream, use of surrounding land, location with respect to urban areas and others.
11. Chlorination/Dechlorination
A Norweco dry chlorination/dechlorination station can be installed to
provide disinfection of the wastewater treatment plant effluent before
final discharge. The station includes a flow regulated proportional feeder
dry chlorinator unit, a proportional feeder dry chlorination unit and
precast concrete contact chamber.
The chlorinator and dechlorinator units are mounted within a precast concrete contact chamber to provide in excess of 45 minute retention of design flow or 15 minute retention of peak factor three flow.
12. Sludge Drying Beds
Beds for
drying excess sludge are almost always provided for plants that have
sludge holding chambers. Sludge drying beds are sized to provide one
square foot of drying surface per 100 gallons of treatment plant capacity.
The beds are made up of 12 to 18 inches of filter media with underdrains;
the sludge goes down into the underdrains which lead back to the inlet of
the treatment facility. The beds have precast or poured-in-place concrete
walls and allow 24 inches of storage space above the surface of the sand.
Usually two beds are provided and used alternately.
13. Lift Stations
Wastewater pumping
stations or "lift stations" are often used ahead of or behind a wastewater
treatment system. They are used to pump the incoming sewage up to the
treatment plant if it is settling higher than the inlet sewer or to lift
the plant effluent to a point of discharge higher than the treatment
facility outfall. Because of the degree of reliability needed with
wastewater pumping station, two alternating pumps are installed in all
lift stations.
The basic lift/dosing station consists of a precast concrete wet well with full access steel doors and is equipped with duplex alternating pumps, mercury level control switches, electrical alternator, flashing light high water alarm and all necessary internal piping, valves and fittings.
14. Electrical Controls
Electrical controls are mounted within a weatherproof Nema 4x fiberglass
enclosure to protect electrical equipment. The enclosure has
two coats of weather resistant paint and is equipped with a master-keyed
padlock.
Electrical controls are 230 volts single phase for plants 20,000 gpd and lower, 230 volts three phase for larger plants. The motor control center includes an across-the-line switch type combination starter, thermal overload protection, circuit breaker and toggle selector switch for each blower unit. The motor control center is securely mounted on top of the wastewater treatment facility adjacent to the mechanical equipment enclosure. Wiring connections form the motor control center to each motor is installed in a liquid tight flexible conduit.
15. Mechanical Equipment
The
air required for the extended aeration process and airlift operation is
provided by rotary positive displacement type. Each blower unit is
provided with an inlet air filter/silencer, flexible discharge coupling
and is connected to the air header assembly by a main supply manifold
complete with a discharge pressure relief valve. Wastewater treatment
plants of this design which consist of two parallel arrangements of
aeration and clarification tanks in series are equipped with separate and
isolated blower units for each series of tanks provided. When a standby
blower unit is provided, discharge check valves shall be included for each
blower unit. Blower connection to the drive motor utilizes an industrial
power transmission drive system.
Mechanical equipment is mounted in a non-corrosive weatherproof enclosure. Each enclosure is constructed of heavy fiberglass for maximum protection and durability. The enclosure consists of a reinforced base and a steel reinforced hood section.
16. Galvanized Metal Grating
Tank
openings are protected with galvanized metal grating padlocked in
position. To permit easy removal and handling, individual grating sections
do not weigh more than 50 pounds each.