PROCESS CONTROL SOFTWARE FOR THE CONTROL OF HYDRAULIC RETENTION TIME IN A BIOLOGICAL REACTOR hgS>G£IPTfC7M

The present invention relates to control software required for the operation of a balance tank reactor as described by Olaopa Process in European Patent No. 1196354 and US Patent No.: 6562237.

Conventionally, biological nutrient removal reactors used for the reduction/elimination of nitrates and/or phosphates operate on 'Archimedes principle' i.e. any flow into the inlet of the reactor displaces similar flow on the outlet. The implication of this arrangement is that hydraulic retention time within the tank can only be defined for a particular flow. At all other flows beside the defined flow the hydraulic retention time is variable.

Olaopa Process describes a biological reactor for the reduction/elimination of nitrogen and/or phosphates incorporating balancing of reactor' s constituents in such a way as to maintain a constant hydraulic retention time (HRT) within the tank under variable flow regime. To achieve this, the outlet from the reactor is located below top water level and the outflow can be controlled with a pump or control valve.

In operation, increases in the hydraulic level within the tank can be picked up by a level sensor and the information translated to mean increasing or decreasing flow at the inlet. The software according to the present invention will then trigger appropriate response in the outlet pump to compensate or trigger an increase/decrease in valve opening to compensate to maintain a pre-determined hydraulic retention time. The desirable control condition would be reached when the incoming flow Q is equal to control volume (V) divided by the HRT. It will also be possible according to the present invention for the software to impose variable hydraulic retention times within the reactor to impose variable hydraulic retention times on different flows; for example for a specified flow regime it might be the case that a hydraulic retention time is imposed on the reactor; beyond these values the retention time can be varied to suit process requirements. In municipal wastewater treatment it is usually the case that flows within certain range in a combined sewer are interpreted as coming from normal diurnal variation. Seasonal fluctuation causing seasonal increase in flows will require different control on the outflow pump or valve. This is because increases from stormflows are accompanied by lower organic concentrations and might require lower retention times within the reactor.

In all known cases of biological nutrient removal, anoxic/anaerobic reactors are defined on the basis of the influent displacing similar volumes on the outlet. Olaopa Process (Pat.No.rEP 1 196354 & US 6562237) referred to in this invention is the only anoxic/anaerobic reactor allowing for variable flow rates on the inlet and controlled flow on the outlet of the reactor.

It is an object of the present invention to provide a computerised programme software to control the outflow from an anoxic and/anaerobic biological reactor using a pump or actuated valve capable of variable opening.

It is also an object of the present invention to provide a control panel that will incorporate a mix-ratio for the reactors influents into the same control panel housing the hydraulic retention time control.

The importance of this aspect is in that in the removal of nitrogen from municipal and industrial wastewater treatment, removal rates are dependent on re-circulation

rates of nitrified mixed liquor coming from an aerobic biological environment into the nutrient removal reactor as defined in Pat. EPl 196354 & US 6562237. Integration of a mix-ratio control software in the same panel as the software for the control of hydraulic retention time allows plant operators the flexibility to control plant performance (see fig. l).

In addition to nutrient removal, variability of dissolved oxygen concentrations in the re-circulating nitrified influent into the reactor can allow for biological oxidation of sulfides, mercaptans and other malodorous compounds contained within the same wastewater. Doing this eliminates the need for chemical treatment of wastewaters for odour removal and dissimilatory reduction of nitrates.

According to the present invention there is provided a control panel incorporating computerised software allowing variation of hydraulic retention time within a biological reactor primarily used for the dissimilatory reduction of nitrates to nitrogen. The control panel may also have incorporated into it computerised software allowing variation of mixing ratio of the influents into the biological reactor typically crude wastewater and nitrified re-circulating mixed liquor.

Preferably based on process requirements, an operator will define a hydraulic retention time and mix-ratio on the control panel. Upon pressing 'START' on the panel the computer programme will control the volume of nitrified effluent to be mixed with the crude wastewater to the treatment plant and the hydraulic retention time within the biological reactor.

As an example of how the software can work, a hydraulic level sensor will read the top water level in the reactor, calculate the volume of the liquid in the reactor and use the reactor volume and selected hydraulic retention time to set a pumping flow rate out of the reactor. If the level in the tank rises there will then be a corresponding increase in pumping rate and vice versa. This will be sufficient if the software is for the control of hydraulic retention time only. In the case of the control panel shown in fig. l the mixing ratio of the re-circulating influent and crude wastewater influent is selected and the desirable hydraulic retention time (HRT) is also selected. The incoming flowrate and desirable HRT is used to calculate appropriate hydraulic level in the reactor. Where the desirable hydraulic level does not match existing hydraulic level in the reactor, the discharge pumping rate can be increased or decreased. This programme can be used for the control of a flow mix into the reactor (re-circulating nitrified influent and crude wastewater) and maintaining a specified hydraulic retention time under variable flow regimes.

In the case of a valve control where an actuated valve is used to control the outflow from the biological reactor, the volume of liquid in the reactor and the selected hydraulic retention time will be used to calculate the percentage opening of a control valve located on the discharge pipework from the reactor. The percentage opening is derived from Orifice principle' that says that for hydraulic reactors discharging under gravity, the hydraulic level is related to the discharge flow rate by this relationship: Q = Cd(2gA) ^1/2 where A is the cross-sectional area of the discharge pipe and Cd is a coefficient of discharge. Desirable discharge flow rate can then be calculated for all flow ranges and the valve opening adjusted to suit.

It will be understood that the control panel can contain either or both of the controls described i.e. hydraulic retention time only or hydraulic retention time and mix-ratio.

The examples above show variations of the software that can be imposed on the reactor. It will be understood however that the present invention will cover any and all software programmes that seek to impose a balancing regime or control the top water level in aerobic, anoxic/anaerobic biological reactors.

It will also be understood that various alterations and modifications may be made to the above control panel without departing from the scope of the invention either by inclusion of other controllers, indicators as typified by inclusion in a Mains Control Centre (MCC), Programmable Logic Control (PLC) panel, SCADA etc. and that the invention is applicable to both municipal and industrial wastewater treatment plant.

It will also be understood that where the anoxic and/or anaerobic reactor are compartmentalized in the same reactor as the aerobic reactor, the control panel according to the present invention remain applicable to the simultaneous control of mix ratios into the anoxic/anaerobic compartment(s) and any control of hydraulic retention time within the tanks.

It will also be understood that the control panel can be applied to the aerobic compartment of any biological reactor.