Chapter 3 Operational Suggestions

3.1 Parameters to Be Monitored by the SCADA System

Oxidation reduction potential (ORP), dissolved oxygen (DO), pH, and alkalinity are parameters that should be monitored by the Supervisory Control and Data Acquisition (SCADA) system. Manufacturers determine what parameters can be monitored and controlled by the SCADA system. Monitoring of certain parameters is important, and the ability to adjust these parameters from a remote location is ideal. The operator needs to be able to add chemicals to raise the alkalinity and subsequently the pH. The set point should be an alkalinity value rather than pH- based. The operator should have the ability to fully control (i.e., modify) the plant-operating parameters, such as (but not limited to) cycle times, volumes, and set points.

SCADA is a computer-monitored alarm, response, control, and data acquisition system used by operators to monitor and adjust treatment processes and facilities.

Alkalinity monitoring and addition ensures that a pH of less than 7.0 does not occur. Nitrification consumes alkalinity, and with a drop in alkalinity, pH also drops. If a plant has adequate alkalinity, pH does not change, so it does not need to be raised. Chemicals that raise alkalinity, such as sodium bicarbonate and soda ash, are recommended over sodium hydroxide. Sodium hydroxide does not raise alkalinity; it does raise pH. See section 2.1.3.1 for a discussion of the pros and cons of various chemicals used to increase alkalinity.

The management of both pH and alkalinity are critical to the effective operation of an SBR. Sufficient alkalinity must be present to allow complete nitrification and result in a residual of at least 50 mg/L in the decanted effluent. The pH must be maintained in a manner to prevent it from falling below 7.0 in the reactor basin. Based on the characteristics of the wastewater, designers should carefully consider the need for both alkalinity and pH management.

For plants that nitrify and denitrify, ORP monitoring is desirable. ORP is the measure of the oxidizing or reducing capacity of a liquid. DO varies with depth and location within the basin. ORP can be used to determine if a chemical reaction is complete and to monitor or control a process.

Operators need the ability to make changes that will modify these readings to achieve appropriate nutrient removal. ORP readings have a range and are site specific for each facility. General ranges are: carbonaceous BOD (+50 to +250), nitrification (+100 to +300), and denitrification (+50 to -50).

On-line dissolved oxygen meters are very useful in SBR operation. They allow operators to adjust blower times to address the variable organic loads that enter the plant. Lack of organic strength reduces the react time during which aeration is needed to stabilize the wastewater. DO probes can be used to control the aeration-blower run time during the cycle, which in turn reduces the energy cost of aeration.

Oxidation Reduction Potential – ORP

ORP measures the electrical potential required to transfer electrons from one compound or element to another compound or element. ORP is measured in millivolts, with negative values indicating a tendency to reduce compounds or elements and positive values indicating a tendency to oxidize compounds or elements.

It is desirable to locate DO, pH, and/or ORP probes in a place that can be reached easily by operators. These probes often clog or foul and need cleaning and calibration. If they are not easily accessible, proper maintenance may not occur.

The plant operator should have the knowledge and the ability to program the SCADA system to increase or decrease blower speed. Allowing the operator to adjust the blower speed, through the SCADA system, gives the operator much more control over the DO in the SBR.

3.2 Cold Climate Adjustments

In general, sewage temperatures are above freezing, but the batch mode of operation exposes the SBR basin to cold winter temperatures. The long, cold ambient air temperature in winter cools the content of a basin below the optimal temperature of 20-25 ºC, which is the ideal temperature for advanced treatment to occur.

SBRs in the northeastern United States should respond appropriately against extreme cold temperatures. Where practical, basins for small or very small systems or facilities should be housed in a garage-type structure to ensure that there is no freezing. Larger basins can also be covered to minimize heat loss; however, when covering basins, ensure that adequate access for maintenance is provided. Consider maximum use of earthen-bank insulation. Exposed piping should be wrapped in heat tape and insulated to protect from freezing. Consider implementing provisions to minimize the freezing of discharge pipes, decanter valves, and chemical lines. Provisions should be considered to minimize ice buildup on decanters. Controllers should be placed in dry areas that are not exposed to extreme temperatures.

The smaller the design of a plant, in terms of flow, the more problematic temperature becomes. The smaller the design flow, the greater the likelihood that between 11:00 PM and 6:00 AM there will be little or no flow. This contributes to a loss of heat from the SBR basins to the surrounding atmosphere. Plants that have flow throughout this time frame lose less heat, because the incoming flow sustains higher temperatures.

3.3 Sampling

3.3.1 Proper Sampling Points

As with all wastewater treatment plants, SBR samples are collected and analyzed for both process control and compliance reporting. Sampling locations must be carefully considered. SBRs that utilize influent-equalization basins have more representative flowpaced composite samples because the discharge is consistent in volume. In other words, flow equalization and true batch reactions allow for easier composite sampling because the same volume is entering and exiting the basin during each cycle.

Twenty-four-hour effluent composite samples should be flow-paced and include samples collected at the beginning and end of each decant event.

3.3.2 Parameters to Monitor

Numerous parameters can be monitored for process control. Testing and monitoring of process control parameters requires planning and organization so that variances from the targeted performance goals are easily recognized. A list of typical process control parameters is provided in Appendix A.

3.4 Solids Retention Time – SRT

Solids retention time is the ratio of the mass of solids in the aeration basin divided by the solids exiting the activated sludge system per day. Exiting solids are equal to the mass of solids wasted from the system plus the mass of solids in the plant effluent.

Ensuring an adequate SRT is very critical to the SBR biological nutrient-removal design process. The design SRT for nitrifying systems should be based on the aeration time during the cycle, not the entire cycle time.

3.5 Sludge Wasting

Sludge wasting should occur during the idle cycle to provide the highest concentration of mixedliquor suspended solids (MLSS). The plant should be operated on pounds of MLSS and not concentration.

Sludge from the SBR basins can be wasted to a digester and/or holding tank for future processing and disposal. The digester-tank and sludge-holding-tank capacity should be sized appropriately, based on the sludge treatment and disposal method.

Supernatant from the sludge digester and/or holding tank should be returned to the headworks or influent equalization basin so that it will receive full treatment. The facility should be designed so that the supernatant volume and load do not adversely affect the treatment process.

A high-level alarm and interlock should be provided to prevent sludge-waste pumps from operating during high-level conditions in the digester and/or holding tanks. Controls should be provided to prevent overflow of sludge from digester tanks and/or holding tanks.

3.6 Troubleshooting

A number of troubleshooting tips are contained in Appendix B.