Chapter 2 Design Guideline

2.1 Preliminary/Primary Treatment

Preliminary treatment includes screening, grit removal, and flow monitoring. Primary treatment includes sedimentation and floatation. SBRs generally do not have primary settling tanks; therefore, effective removal or exclusion of grit, debris, plastics, excessive oil or grease, and scum, as well as screening of solids should be accomplished prior to the activated-sludge process.

2.1.1 Screening Influent Wastewater

Bar screens or mechanical screens should be used instead of grinders or shredders. Screening influent wastewater is a positive means of removing rags, sticks, and other debris before they can enter the treatment process. Grinders and shredders pass this material into the SBR where it can become woven together, making it difficult to remove. Removing debris from the wastewater stream before it reaches the basins is beneficial to both the treatment process and the settling phase—excess debris is not present to interfere with the solids that need to settle, resulting in a high-quality sludge blanket. Screens also provide protection for the pumps.

2.1.2 Influent-Flow Equalization

Flow equalization is critical where significant variations in flow rates and organic mass loadings are expected. Flow equalization is also important if a plant is expected to receive a significant amount of septage or is taking in a significant amount of industrial wastes. Flow equalization is strongly recommended when a plant needs to achieve nitrification and denitrification. It is important to note, however, that the size of the influent equalization basin must be carefully considered because an oversized basin can cause negative downstream-treatment-process impacts. A plant utilizing an influent equalization basin will be able to have a true batch reaction.

Influent-flow equalization benefits the SBR process in the following ways:

• Allows for a smaller SBR-basin size because it allows for storage until the process cycle is complete.

• Allows for one basin to be taken off line for maintenance or for seasonal variations. Routine maintenance is necessary for all tanks. For plants that have seasonal variations, taking one basin off line is cost-effective due to a reduced need for electricity, staff hours, and tank maintenance.

• Allows for scum and grease removal at a single point before it enters the SBR tank. Entrainment by mixing should not be the sole means of scum control. A mechanism or process for removing scum, grease, and floatables should be provided in the equalization tank.

• Allows plants that must denitrify to ensure that an adequate amount of carbon is available in the denitrification fill phase.

• Allows for an equal flow volume into the basin, keeping the food to microorganism ratio (F/M) fairly stable.

With the use of influent-flow equalization and bar or mechanical screens, the wastewater stream entering the SBR is free of grease, scum, rags, sticks, floatables, and other debris, making it easier to treat.

As stated previously, each SBR design is unique and in some situations influent-flow equalization basins may not be required to obtain optimum treatment. Examples of where influent-flow equalization is not needed include (but are not limited to) plants designed with three or more SBR basins and plants that do not need to nitrify and denitrify.

If a plant is operating with a two-basin system without influent-flow equalization, then it should have an adequate supply of essential spare parts onsite. This will allow broken components to be returned quickly to service without the need to wait for back-ordered parts.

The influent-equalization basin should have a form of agitation or mixing to keep the solids in suspension. A mechanical-mixing unit can be used for this purpose. Maintenance on this basin should be minimal as the solids are in suspension due to the agitation; however, a means to bypass the equalization basin and to dewater the basin should be provided. Pumps that direct influent to the SBRs should be in duplicate. Influent-flow equalization should be designed to hold peak flows long enough to allow the active treatment cycle to be completed.

2.1.3 Piping for Alkalinity Addition

Ideally, facilities should provide piping for adding alkalinity at both the influent- equalization basin and the SBR basin. It is also desirable to be able to measure alkalinity at each location. Alkalinity addition should be based on the amount measured during the decant phase, not on incoming flow. Alkalinity should be kept in a range of 40-70 mg/L as CaCO 3 prior to the decant phase to be sure the nitrification cycle is complete. Consider implementing a method of alkalinity addition even if a facility is not designed to nitrify.

Alkalinity

Alkalinity is a measure of how much acid must be added to a liquid without causing a great change in pH. Expressed another way, alkalinity is the capacity of water or wastewater to neutralize acids. This capacity is dependent on the content of carbonate, bicarbonate, and hydroxide in wastewater. Alkalinity is expressed in mg/L of equivalent calcium carbonate (mg/L CaCO 3 ). Alkalinity is not the same as pH because water does not have to be strongly basic (high pH) to have high alkalinity.

When nitrification occurs at SBR facilities, it often occurs during periods of diurnal low flow (e.g., very late evening or very early morning) when a plant is not staffed. If no testing or chemical addition is available to compensate for an alkalinity drop, pH in the SBR unit will drop and cause process upsets.

2.1.3.1 Options for Adding Alkalinity.

Sodium Bicarbonate, a.k.a. Baking Soda (NaHCO 3 ) - Sodium bicarbonate is most often recommended for alkalinity addition because it is not a strong base and it has a pH of 8.3. It is beneficial to alkalinity addition by providing the bicarbonate species at a pH near neutrality.

Sodium Carbonate, a.k.a. Soda Ash (Na 2 CO 3 ) - Soda ash is safer to handle than other alkalis and tends to maintain stable prices over time, hence more and more treatment plants are choosing soda ash for their alkalinity needs. While soda ash is less expensive than sodium bicarbonate, it is generally less effective than sodium bicarbonate and sodium hydroxide. Soda ash is a moderately fast- acting agent, but it generates carbon dioxide, which can lead to foaming problems.

Calcium Oxide, a.k.a. Lime (Ca(OH) 2 ) – Lime is available in various forms and is relatively inexpensive. Lime compounds dissolve slowly and require longer contact times than the other chemical options. The use of lime causes more sludge production due to calcium sulfate precipitation. This results in maintenance problems within the basin, especially with pH, DO, and ORP probes.

2.2 Sequencing Batch Reactor

2.2.1 Basin Design

Ideally, plant designs should have a minimum of two SBR basins and one flow- equalization basin; however, every design is unique and one configuration does not fit all situations. All SBR designs should have a minimum of two basins to allow for redundancy, maintenance, high flows, and seasonal variations. Two basins allow for redundancy throughout the plant. If one basin is off line, the plant is still able to treat influent wastewater because of the equalization basin. If the basin microbiology becomes depleted in one basin, the biomass from the remaining basin can be used to restock the basin with depleted biomass. For this to happen, a means of transferring sludge between the two basins must be provided.

During storm events and high-flow periods, instead of bypassing the basins or blending the stormwater, an additional basin can act as storage, or certain cycles can be shortened. In particular, the react cycle can be shortened under wet-weather conditions because of the diluted flow and the reduced time needed to treat the BOD. With higher flows, the fill phase and the idle cycle can also be shortened. A two-basin design also allows the plant to take one basin off line for draining and cleaning while the pre-flow basin and the one on- line basin remain fully operational.

For plants that have seasonal flow variations, a design that includes two treatment basins and an influent-flow-equalization basin allows one basin to be taken off line during the off season. This is important for seasonal plants, as it can save money by cutting electricity costs and reducing staff hours (fewer hours are spent on overall basin maintenance). The basin that remains on line is able to reseed the biomass in the off-line basin when the influent flow pattern peaks.

2.2.2 Flow-Paced Batch Operation

Flow-paced batch operation is generally preferable to time-paced batch or continuous- inflow systems. Under a flow-paced batch system, a plant receives the same volumetric loading and approximately the same organic loading during every cycle. The SBR basin already has stabilized supernatant in it, which dilutes the batch of incoming influent.

Under a time-paced mode, each basin receives different volumetric and organic loading during every cycle, and the plant is not utilizing the full potential of this treatment method—the ability to handle variable waste streams. After each loading, the plant faces a whole new set of treatment conditions, making the operator’s job more difficult.

Time-paced operation (if you are not adjusting the cycle time) can lead to under-treated effluent. A plant that receives heavy morning loadings, with a flow pattern that drops off after the first cycle, must deal with two different biologies in the basin unless adjustments are made to the cycle time. For example, one basin could be receiving an early morning load, which has a high organic and volumetric loading. The second basin could be receiving the afternoon loading, which has a lower organic and volumetric loading. Unless the time cycle is adjusted, it becomes difficult to operate under these conditions because the operator is essentially running two separate plants.

Another problem with time-paced operation is that if the plant is required to denitrify, it may not bring in an adequate carbon source needed for the bacteria to strip oxygen from the nitrate. This scenario would be especially problematic during periods of low flow.

For an SBR to be effective, the plant must have proper monitoring, allow operators to adjust the cycle time, and have knowledgeable operators who are properly trained to make the necessary adjustments to the cycle.

Lessons from the Field

An operator with a full understanding of the SBR process and operations can overcome design limitations. For example, an operator at a plant operating under time-paced configuration was able to overcome operational restrictions by reprogramming the programmable logic controller (PLC) so that the decant phase would not be initiated until the high-water level (HWL) was reached. If the basin did not reach the HWL during the cycle it would not decant and would take in the next load until the HWL was reached. It would then complete the cycle and decant. This is not a recommended mode of operation, but it demonstrates how skilled operators with thorough knowledge of how their plant operates can make adjustments to benefit the process. Operators need the flexibility to fully control and operate their plant, since they are the ones who are responsible for it.

2.3 Blower Design

Several smaller blowers are preferable to one large unit. It is not uncommon for SBR designs to incorporate a single blower per basin to provide aeration. However, operational efficiency can be enhanced when plants utilize several smaller blowers, instead of one large blower.

Variable Frequency Drives – VFDs

In wastewater facilities, pumping and aeration account for a majority of energy consumption. These applications are well suited to the use of VFDs. A VFD is an electronic controller that adjusts the speed of an electric motor by varying the amount of power supplied. A VFD varies both the frequency (hertz) and amplitude (volts) of the alternating current waveform. This allows the motor to continually adjust in order to work just hard enough, rather than running full speed all the time. Wastewater facilities that have installed VFDs have seen a 25 percent reduction in energy costs for pumping and aeration, as well as increased equipment life and decreased maintenance costs.

When a single blower per basin is used, it should be sized to provide maximum aeration under worst-case conditions. These conditions typically occur in the summer months, when higher temperatures decrease the amount of oxygen that can be dissolved in wastewater. For facilities that utilize a single blower per basin, a variable frequency drive should be considered.

In a plant configured with only one blower per basin, it is difficult to scale back on the aeration provided. With multiple smaller blowers, units can be taken off line when maximum aeration is not required. This results in electrical cost savings.

Fine-bubble membrane diffusers are preferable to coarse-air bubble aeration. Fine-bubble diffusers transfer more oxygen to the water due to increased surface area in contact with water. The same amount of air introduced in a big bubble has less surface area in contact with water than an equal amount of air divided into smaller bubbles. The amount of surface area in contact with water is proportional to the amount of oxygen transferred into water. Depth of aerators also plays a part in oxygen transfer, due to contact time. The deeper the aerator, the longer it takes for the bubble to come to the surface. Aerator depth is deepest when a tank is filled to the high-water level. If a plant is utilizing time-paced batch reactions, aerator depth is not optimal and oxygen contact time is not maximized.

Blowers in multiple units should be sized to meet the maximum total air demand with the single largest blower out of service.

2.3.1 Decanting

During the decant phase, operating under a flow-paced batch operation, no more than one- third of the volume contained in the basin (i.e., the tank contents) should be decanted each time in order to prevent disturbance of the sludge blanket. The decant phase should not interfere with the settled sludge, and decanters should avoid vortexing and taking in floatables. The problem with decanting more than one-third is that it increases the chance the plant to run optimally, it is important that the decant volume is the same as the volume added during the fill phase. The length of the decant weir can have an impact that is very similar to that of the over-flow weir found in a clarifier. The flux (upward forces) caused by the discharge of the decant creates an upward force that may pull poorly settled solids up and out the discharge.

2.3.2 Bottom Slope

All basins should have a sloped bottom with a drain and sump for routine tank maintenance and ease of cleaning. Slope rectangular basins slightly to one corner to allow for hosing down the unit. Circular basins should be sloped toward the middle for maintenance. All SBR designs should include a means for completely emptying each SBR unit of all grit, debris, liquid, and sludge.

Lessons from the Field

Tank maintenance is an intensive process and can use up valuable maintenance hours. In one situation, a 0.75 million gallon, flat-bottomed, rectangular basin took close to 24 man-hours to dewater and clean. By slightly sloping the floor to one corner, this process would have been much less labor intensive and would have saved significant maintenance time.

2.4 Post Basin

2.4.1 Post-Basin Effluent Equalization

Post-basin effluent equalization smoothes out flow variations prior to downstream processes, such as disinfection. By providing storage and a constant smooth flow, the disinfection process will be more effective. If post-flow equalization is not utilized, the effluent might not receive the designed amount of treatment. Post-basin effluent equalization also allows downstream processes to be sized smaller, since the flow from the basin is metered out and does not hydraulically surge the downstream processes.

Effluent equalization also ensures that there are not large variations in operating ranges for the metering pumps and the chlorine analyzers. Ideally, the basin should be of sufficient size to hold a minimum of two decantable volumes. There should be a means of returning the liquid from the post-flow equalization basin to the headworks if a poor decant occurs. These basins should also have a means of removing solids from the bottom of the unit, such as a sloped bottom with a drain or sump.