Phosphorous removal – Optimisation of settlement-enhancing chemicals

Introduction:

Municipal and Industrial wastewater treatment plants receive phosphorous from human waste, food and certain soaps and detergents. Removing it during the process and controlling its discharge in final effluent is a key factor in preventing eutrophication (algal blooming) of surface waters. Its presence potentially causes many ecological, economic and health problems (e.g. algal toxins in drinking water). Phosphate removal is currently achieved largely by chemical coagulation/precipitation, which is expensive and causes an increase of sludge volume by up to 40%. Any chemical which enhances settlement will tend to remove phosphorous from the fluid while growth of biomass also removes phosphorous as it becomes part of the growing microorganism population, so both settlement and optimised biomass growth can be used to remove phosphorous from the final effluent.

Regulatory authorities are reducing the phosphorous consents for final effluent so optimisation within the wastewater process is critical.

The balance within a wastewater process:

Treatment plants produce primary sludge and surplus activated sludge and one method for disposing of, particularly solid waste, involves anaerobic digestion.  Anaerobic digestion produces methane which can be captured and used, for example in a CHP (combined heat and power) engine to supply some of the electricity power requirements of the plant, the objective of many sites being a neutral carbon balance.  It can therefore be advantageous to increase sludge production by adding more settlement enhancing chemicals, although there is an economic cost involved.

Conversely, the potentially negative effect of removing too much phosphorous from the primary effluent is that phosphorous nutrient which is required in the growth of microorganisms and clarification of effluent by the sludge lanes is limiting to the process.

Current techniques for determining the required amount of settlement chemicals are relatively crude, for example based upon visual appearance of the effluent, possibly supplemented with a measurement of phosphorous (phosphate) level. There is a need for improved techniques for controlling the dosing of settlement enhancing chemicals in a wastewater treatment plant.

Manometric Respirometry (Shepherd System) and chemical dosing:  

The trade-offs described above can be mitigated by aligning the dosing of settlement-enhancing chemicals with the biochemical oxygen demand (BOD) of the plant.  To put this into effect one could measure the BOD of the influent to the plant and then dose accordingly, for example to maintain a target ratio of BOD to phosphate.

This approach does not however work well, because it is incorrect to consider the activated sludge region of the plant merely as a combination of influent and micro-organisms. Instead, the activated sludge liquor becomes attuned to the feed stock, the feed stock affecting the different populations of micro-organisms which grow, and the different micro- organisms in turn affecting the activated sludge liquor so that there is a complicated interaction between the feed stock and micro-organisms. There is a need to account for the activity of the biomass into which the influent is provided.

Bactest’s Shepherd System measures the biochemical oxygen demand (BOD) in activated sludge downstream of the location where the settlement enhancing chemicals are added.  A Shepherd measurement in this location can then be used to control the chemical dosing upstream of the measurement location, dosing the influent or alternatively the RAS (returned activated sludge) feed into the activated sludge lane.

The target to be achieved by the dosing depends on the type of plant and its operating conditions however the general principle is controlled dosing to a desired ratio of phosphorous to biochemical oxygen demand.  The amount of chemical is adjusted to the target ratio and in proportion to the flow rate.

The dose of chemical (to achieve a desired target level of phosphorous) may be determined by an initial calibration process involving a lab measurement to identify the correct level of phosphorous, in combination with data from a chemical manufacturer, which defines the amount of chemical which must be added to achieve a particular BOD: phosphorous ratio.

The phosphorous needs of the growing micro-organisms can be inferred from a measurement of the biochemical oxygen demand (or a parameter dependent upon this) in the activated sludge, and knowing this, the phosphorous requirements can be estimated and then the dosing upstream controlled to meet these.  This helps to avoid having either too little or excess phosphorous, both of which would be suboptimal.  Furthermore, when the growth is optimised the sludge production can be effectively maximised without excess phosphorous in the plant output, thus facilitating feedstock extraction.

Typical ratio for BOD:phosporous:

BOD is a standard measure (reported in mg O2/litre of sample) calculated by incubation of a sample over five days in a laboratory. This technique is impractical for the closed loop control however Shepherd’s online test in just one hour to create a BOD5 proxy makes it ideal for this application.

The optimum ratio of BOD5 to phosphorous depends upon the units of measurement, however for a total phosphorous level measured in milligrams per litre, a target BOD to phosphorous ratio is typically in the region of 100:1.  Optimising the ratio and measuring in near real time saves chemical costs and potentially facilitating the use of improved but more expensive materials such as polyelectrolyte. Such control promotes a good, healthy biological process and reduces the risk of overdosing with chemicals, which might otherwise need to be corrected by adding a costly carbon source such as methanol.

Software calculates BOD and phosphorous demands in real time, allowing control systems to accurately deliver a ratio of BOD:P, e.g. 100:1. Compensation can be made within the software for different settlement enhancing chemicals where more than one is employed, based on the manufacturers product/batch data, adding their respective influences on phosphorous and settlement rate.  The system is integrated, has an audit trail and the system is viewable over secure internet.

Added value:

Optimising the nutrient, creating a healthy, rapidly settling floc has the added advantage of reducing dewatering costs of the SAS process.

Contact the Author – Derek Price

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