(a) Pretreatment. Where primary clarification is not provided, screening of the raw sewage to remove debris larger than 3/4 inch (1.9 cm) shall be provided. The screened material shall not be returned to the plant process. Where primary clarifiers are not provided, cleanouts, grinders, or other similar provisions shall be made in the return sludge piping.
(b) Loading rates. Activated sludge systems shall be designed to accommodate peak day loadings at the design year. Permissible loadings are presented in the following table. Where raw sewage BOD 5 is less than 200 mg/L, detention times may be reduced. - (i) Conventional, including complete mix, plug flow, step aeration
| | Average Day |
| Detention (*) hrs, | Following primary clarifiers | 6 minimum |
| Without primary clarifiers | 9 minimum |
| Organic Loading: | lb/1,000 cu ft/day (kg/1000 m3d) | 35 maximum (560 |
| MLSS, mg/L | | 1,000 - 3,000 |
- (ii) Contact stabilization.
| Detention (*) hrs, Contact Zone | | 0.5 - 3 |
| Sludge Stabilization Zone | | 6 minimum |
| | Average Day |
| Organic Loading (**) | lb/1,000 cu ft/day | 50 |
| (kg/1000 m3d) | (800) |
| MLSS, mg/L Contact Zone | | 1,000 - 3,000 |
| Sludge Stabilization Zone | | 5,000 - 10,000 |
(c) Number of basins. For all design average flows in excess of 0.1 mgd (378 m3/d), two or more aeration basins shall be provided. For flows less than 0.1 mgd (378 m3/d), one aeration basin may be provided if the aeration devices can be readily removed while the basin is in operation.
(d) Configuration. The basin configuration shall promote mixing, transfer of oxygen, and minimize stagnant zones.
(e) Freeboard. The walls of the aeration shall extend above the normal water surface to provide a minimum freeboard as follows:
| Minimum inches | Freeboard (*) cm |
| Diffused air | 18 | 45.7 |
| Surface aeration | 48 | 121.9 |
| Submerged turbine | 18 | 45.7 |
| Brush aeration, less than 10 feet from aeration device | 48 | 121.9 |
| Brush aeration, 10 feet or more from aeration device | 18 | 45.7 |
| Surface aeration, where aeration | 36 | 91.40 |
| is 30 or more feet from basin wall | | |
(*) Vertical walls. For sloped walls, the runup effect shall be considered.
(f) Inlet and outlet conditions. Inlets may be submerged and shall be baffled or directed away from the outlet to minimize shortcircuiting. Outlets shall be of the overflow type to discourage buildup of foam and floatables on the aeration basins. Pipe and channels shall provide a minimum velocity of 0.5 fps (0.15 m/s).
(g) Aeration requirements. - (i) Carbonaceous BOD. When it can be shown that nitrification will not occur in the activated sludge process, the aeration devices may be sized to meet only the carbonaceous oxygen demand. The oxygen provided by the aeration device shall be selected to be adequate for the projected maximum day loading. In the absence of other data, an oxygen requirement of two (2) times the average design day BOD5 to the aeration basin shall be used.
- (ii) Nitrification. Where nitrification is required to meet the effluent requirements or where the process cannot be operated to prevent nitrification, the aeration requirements will be selected to provide oxygen for both carbonaceous BOD and nitrification on the projected maximum day loading. In the absence of other data, an oxygen requirement of two times the average design day BOD5 plus 7.5 times the average day ammonia nitrogen to the aeration basin shall be used.
- (iii) Minimum dissolved oxygen. Oxygen supply shall be selected to transfer the design quantity during the maximum day loading while maintaining an aeration basin dissolved oxygen of 2.0 mg/L. The oxygen supply shall be designed for the specific site considering all factors that affect oxygen transfer efficiency.
(h) Mechanical aeration. Mechanical surface aerators shall be designed to maintain all organics in suspension, enhance the oxygen transfer capability of the unit, and minimize mist and spray that escape the basin. Drive units shall be protected from freezing mist and spray.
(i) Diffused aeration. - (i) Diffuser requirements. The number and location of diffusers shall be selected to distribute the design air quantity for efficient aeration and mixing. Diffusers in a basin shall be grouped on control valves to permit varying the air supply to different parts of the basin. Oxygen transfer efficiencies used for design purposes shall be conservatively selected, based on experimentally determined transfer rates of generically similar diffusers. The effect of transferring oxygen to wastewater, in lieu of water, and the effect of altitude shall be considered. The aeration basin mid-depth shall be used to determine the oxygen saturation concentration. Differential head loss to individual diffuser inlets shall not be more than 0.2 psi (14 gm/cm2).
- (ii) Blower requirements. Blowers shall be sized to provide the air requirements for the aeration basins and other plant uses of low-pressure air. The inlet air to the blowers shall be filtered or otherwise conditioned to effectively remove dust and other particulate material. Removal of particulate material for fine bubble diffusers shall be designed for 95 percent of 0.3 micron. Filters designed for blowers shall be easily replaceable. Blower intakes shall be located to avoid clogging from drifting snow. Blowers shall be housed. The housing shall be ventilated to prevent more than a 15° F (8° C) temperature rise with all blowers operating, excepting the standby blower. The housing, blowers, and blower piping shall be arranged to permit removal of individual blowers while all other blowers are operating. Noise attenuating materials shall be used in the building interior. Blower systems shall be designed to permit varying the volume of air delivered. Blower motors shall be of a size to operate the blower throughout the range of ambient air temperatures experienced at the plant site.
(j) Sludge recirculation and waste. - (i) Rates. Sludge recirculation from the secondary settling basin to the aeration basin shall be variable within 25 to 100 percent of the average design flow. Sludge wasting from the activated sludge process may be from the mixed liquor or the return sludge. Sludge wasting shall be variable to enable wasting ½ of the total system solids in one day to zero wasting.
(k) Equipment requirements. - (i) Return sludge. Return sludge pumping shall be variable. The return sludge rate from each secondary settling unit and the rate to each aeration basin shall be controllable. Pumps shall be housed in heated, ventilated space. The pump floor shall be sloped and drained. Valves shall permit isolating each pump. Pumps and piping shall be arranged to allow ready removal of each pump. Check valves shall be provided where backflow through the pump could occur. Check valves shall be located in the horizontal.
Pump suction and discharge shall be three (3) inches (7.6 cm) minimum diameter. Sludge piping diameter shall be four (4) inches (10.2 cm) or larger. Cleanouts and couplings shall be provided in sludge piping to enable cleaning the pipe or to remove pumping equipment. All pipe high points shall be provided with air releases. All sludge piping shall be metallic material. Should air lift pumps be used, the units shall be designed with a minimum of 80 percent static submergence.
- (ii) Waste sludge. If separate waste sludge pumps are provided, the rate shall be controlled by timers or variable speed devices. Pumping units shall be housed in heated, ventilated space, with sloped and drained floors. Pump suction and discharge piping shall be three (3) inches (7.6 cm) minimum diameter. Sludge piping shall be four (4) inches diameter (10.2 cm) or larger, except short, easily removable sections that may be required to maintain velocities above one fps (0.3 mps), or for use in conjunction with meters.
(l) Metering. - (i) Return sludge. For treatment plants having an average day design capacity greater than 100,000 gpd (378 m3/d) the return sludge flow rate from each secondary settling unit and to each aeration basin shall be metered to indicate flow rate. Return sludge metering devices shall be suitable for liquids carrying grease and solids, and shall be accurate to within ±5 percent of the actual flow rate. Meters shall be readily field calibrated by plant personnel. Meters shall be arranged to avoid trapping air.
- (ii) Waste sludge. For treatment plants having an average day design capacity greater than 100,000 gpd (378 m3/d), waste sludge flows shall be metered to indicate and totalize. Waste sludge meters shall meet the requirements described for return sludge meters.
- (iii) Air flow. Low-pressure air used for basin aeration and other plant uses shall be metered. Separate meters shall be used to indicate the flow rate to each aeration basin and to the ancillary uses made of the low-pressure air. Indicators shall be located near the device used to control the air flow rate. Pressure gages shall be provided immediately downstream from each blower and immediately upstream of each aeration basin.
(m) Controls. Facilities for control shall be provided for: - (i) Control of flow split between parallel process units.
- (ii) Control of return sludge flow rate to each aeration basin.
- (iii) Control of waste sludge quantity.
- (iv) Control of air flow rate to each aeration basin.
- (v) Control of air distribution to different zones in aeration basin.
- (vi) Control of energy imparted with mechanical aeration. Facilities for control shall include a meter or device to measure rate and a device to change the rate such as a valve or adjustable weir.
(n) Prefabricated treatment units. Prefabricated activated sludge units shall conform to the applicable requirements described.
(o ) Ancillary facilities. Adequate nonpotable washdown water shall be provided around the aeration basins sludge pumping area and secondary settling basins. Sampling ports, pipes or other access shall be provided on aeration basin inlets, return sludge piping, waste sludge piping and secondary settling basins. Hoisting or other means of equipment removal shall be provided. All subgrade floors shall be drained.
020-11 Wyo. Code R. § 11-14