AQUATREAT ENVIRONMENTAL ENGINEERING LIMITED
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Activated Sludge Plant Solids Management System
Introduction
Control of the biomass (Mixed Liquor Suspended Solids or MLSS) is arguably the most
critical variable when managing and operating an Activated Sludge Plant (ASP).
This is traditionally carried out manually by the plant operators and typically
involves taking a spot sample of the plant MLSS and RAS (Return Activated Sludge)
solids. This process is time consuming for the operator, provides only a limited
snap shot of the plant and is often the sole basis of a decision to modify the SAS
(Surplus Activated Sludge) rate. Furthermore this snap shot of the plant is already
out of date by the time all the tests are complete. Until now real time solids measurement instruments have failed to provide the operator with sufficient confidence to automatically control the plant. These instruments have been prone to drift and have proven themselves useful for trending only but not for absolute measurement of MLSS and RAS solids.
The ASP real time Solids Management System has overcome these shortfalls and is now being accurately and reliably used for ASP plant solids control at sites both in the UK and USA.
How Does the System Work?
The ASP Solids Management System continuously measures, records and controls, either
directly or indirectly, the level of biomass (MLSS) in the plant by determining
the sludge wastage rate within limits set by the operator.The controller records a solids measurement every 15 minutes and stores all historical data and trends for the previous week. This data is used to calculate the exact mass of sludge to be wasted on a daily basis. The controller can be set to waste the sludge whenever the operator desires e.g. continuously or intermittently; either once a day or several times a day. The controller can be used to control an actuated valve, SAS pump (fixed or variable speed) or simply to advise the operator or plant SCADA system on the wasting actions to be taken.
The operator can specify ranges and set alarms to ensure the controller does not waste insufficient nor excessive amounts of sludge. It has a fail safe condition where wasting can revert to a simple predetermined surplusing regime in the event of a loss of signal from the instruments.
Solids Management
The ASP Solids Management System uses a patent pending colour compensating solids sensor. Traditional solids meters (within the wastewater industry) use infra-red (IR) sensors which are prone to drift due to IR absorption by irregularly shaped solids (i.e.MLSS) and any colour present in the water. The solids sensors used by the ASP Solids Management System are not affected by these variables and therefore are not prone to drift in the same way.The solids sensor incorporates a Phased Array Colour Compensating Optical System. The system has three coloured sources (red, blue and green) which sequentially illuminate. During each individual source illumination the instrument calculates the change in absorbance of each colour and corrects the solids reading in relation to the calibration setting of that colour. The final reading is a direct measurement of solids corrected for any colour in the sample.
The System in Operation
In order to control an Activated Sludge Plant the controller measures the MLSS concentration
in the plant and the solids in the SAS line. The system also requires the SAS flowrate which
can be taken as an input from existing plant instruments.The plant dimensions are input to the controller at the commissioning stage. Coupled with the basic real time measurements the controller will accurately and reliably control your ASP.
The graph shows the difference the controller made once it was brought on line. The green and black lines depicts manual and automatic control respectively.
Benefits of the ASP Management System
- Stable and robust ASP resulting in consistent production of high quality effluent with minimal demands on operator time
- Operator has more quality information for running and troubleshooting the plant and is not wasting a lot of time on “spot” sampling and consequent “reactive” control
- Reduced aeration costs due to more precise control of MLSS i.e. reduced endogenous respiration
- Elimination / reduction of foaming and/or sludge bulking in final settlement tanks due to improved solids control
- More consistent surplus sludge concentration resulting in more stable sludge processing
- Reduction in polymer consumption cost for sludge thickening due to more consistent SAS
- Improved plant stability allows more reliable prediction of sludge disposal and hence tanker requirements
Aquatreat Environmental Engineering Limited is a company registered in England and Wales
Company Number 04675334, VAT Number 812 5213 69.