Optimisation saves energy

New technology has been developed that allows the quick and accurate determination of the critical dissolved oxygen concentration for activated sludge. Ken Diamond of Strathkelvin Instruments reviews its implementation at two UK sites

Through respiration, bacteria produce the energy that facilitates the biodegradation of organic compounds in wastewater treatment facilities. Bacteria respire by diffusion across their cell wall where oxygen diffuses from high external concentration to low internal concentration.

Bacteria can only biodegrade at a specific maximal rate. So, determining the critical oxygen point at which a particular biomass cease to operate at their optimal level allows operators to set the dissolved oxygen (DO) levels of the control system (where they have this capability). Having aeration systems set above this critical oxygen point is a waste of energy and money.

Using respirometry techniques developed by Strathkelvin Instruments, a simple oxygen uptake rate (OUR) protocol can be used to determine the maximal biodegradation capacity. Further analysis of the critical oxygen point is completed automatically as the level of DO falls below the point where diffusion across the bacterial cells walls starts to be compromised. The critical oxygen point changes from biomass to biomass, and over time for a particular biomass depending on plant conditions.

For several years, aerobic treatment plants have been fitted with sophisticated DO control systems. In general, these control systems have been set up to control DO concentration by rules of thumb. Accurately knowing the critical oxygen point for the actual activated sludge present in the treatment plant allows the optimal set up of the plant control systems.

Depending on the existing control regimes and operating philosophies in place, it was thought that savings in the region of 10-40% of aeration energy could be saved.

Currently the average saving is 33%.

Making any change to the operation of a WWTP always requires careful management of risks and operators need reassurance that the plant will remain in compliance. The process engineering and control system on the plant must be fully understood and inspected.

The relationship between the dissolved oxygen control set point, the actual measurement and the actual DO concentration at all areas in the treatment basin must be fully understood. In this way, the correct system dissolved oxygen set point can be determined following measurement of the critical oxygen set point.

Completing the plant survey can be carried out during a one-day site survey.

Subsequent review visits can normally be carried out within two to three hours.

Strathkelvin Instruments has recently carried out the following standard assessment on two UK plants:

1. Current condition established including:
  • Critical oxygen point determination more than ten separate samples
  • Basin survey to determine how accurate current control systems are and how the control level varies across the basin
  • Microbiological baseline establishment
  • Basin survey to determine biodegradation profile as a baseline
  • Basin physical performance characteristics such as mixed liquor suspended solids (MLSS), sludge age and sludge volume index (SVI)
2. Developing a project plan, which will take a step-wise approach to reducing oxygen control set points and therefore energy consumption within the constraints of the basin oxygen supply system

In addition, each project followed these additional guidelines:
  1. Prioritisation of the utility company treatment facilities on the basis of benefit of change, cost of change, risk of change and the full evaluation of the new technology
  2. Determination of risk for each facility proposed and the necessary management procedures and techniques which would be established
  3. Presentation to stakeholders of the proposed changes, estimated benefits and risk management process. Buy in received from these stakeholders
  4. Detailed site review including treatment regimes, current operating performance, process equipment, and control systems
  5. Determination of critical oxygen points
  6. Establishment of baseline readings for risk management:
    • Microbiological survey
    • Sludge health measured by the specific oxygen uptake rate (SOUR)
    • Toxicity assessment of influents
    • Basin bioactivity profiling using in-basin respirometry
    • Short-term biochemical oxygen demand (BODst) on influent and effluents
    • Mixed-liquor suspended solids readings
    • Sludge settlement index or stirred sludge volume index as appropriate
    • Temperature and acidity
  7. Changes from current control regime to the determined optimal level were then implemented on a stepwise basis with baseline readings at 0.6 above, continuously monitored for at least four sludge ages, prior to making the next incremental stage.
Results
Although the water companies involved did not want their plants to be identified, it can be revealed that in both cases there was no need for investment in additional plant to implement the required changes.

Plant 1: After the initial review and training of local operators, the plant showed the following improvements:
  • 33% reduction in aeration power costs achieved
  • 50% improvement in sludge settling characteristics
  • Treatment volume reduced by 20%
  • Odour improvements particularly at secondary clarifiers
Savings to date are in the region of £130,000 a year for a project total cost of £20,000. Graph 2 shows airflow comparisons before and after the optimisation of plant 1.

Plant 2: After a two-day initial analysis and a further one-day review after a month, the plant was shown to have improved as follows:
  • 20% reduction in energy costs
  • Elimination of significant foaming problems in aeration lanes and at clarifiers
  • Elimination of filamentous bacteria
  • Improvement in presence of higher order biological species in mixed liquors
  • Significant reduction in odour emissions
The data on cost savings is not yet available, but the project cost £17,000.

Analysis at the two sites was carried out using two respirometers form Strathkelvin. The Strathtox (pictured left) is a bench top respirometer with test protocols for several parameters: sludge health, specific oxygen uptake rate (SOUR), critical oxygen point and short-term biochemical oxygen demand (BODst). The necessary laboratory tests can be completed within two days, even for a major plant.

The multiple cell analysis also provides a wealth of data concerning the particular characteristics of the plants biomass. This, combined with the data from biodegradation profiling, provides valuable and precise data for the process scientist and engineer.

Establishing a profile for activity levels in the actual plant basin is extremely effective both as a troubleshooting tool and as a predictor of compliance performance on the plant. This type of on-site profiling was carried out using Strathkelvin Instruments' Bioscope.

With the bioscope open (see lefthand page, image A), the unit is inserted into the treatment system. The technician then waits until the DO reading and the temperature reading stabilises. This normally takes 30 to 40 seconds. Once temperature and DO have stabilised the technician operates the lever to close the respirometry cell (see image B). The system automatically switches on the stirring rotor and records the OUR. The unit can be withdrawn from the system and the settlement characteristics observed - stirred and non-stirred

The control of DO levels in WWTPs has been the subject of considerable technological advancement over the past two decades. Typically, the energy required for aeration purposes in aerobic activated sludge plants is in the region of 35% to 65% of the total energy costs for the facility.

By accurately measuring this value and using it to set up the plant aeration control
systems, energy savings of up to 40% of the overall aeration energy costs can be made. In addition, customers note the following benefits:
  • Maximisation of ammonia biodegradation
  • Improved settlement, reduction of foaming and improvement of microbiology
  • Reduction or elimination of unwanted nitrification
Strathkelvin has shown that, by using a number of features that come from their method of respirometric analysis, and the combined use of two of its products, substantial savings can be made in energy consumption. Improved plant operation can lead to significant reductions in CO2 emissions throughout the secondary treatment facilities of a WWTP.

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