In this week’s blog Oliver Grievson looks at the most basic process of sludge treatment outside what is done on sites in settlement tanks or sludge tanks. He is of course thinking about mechanical thickening or dewatering, and will of course exclude picket fence thickening (it has its place but not in this blog).
When we look at the processes of mechanical thickening and dewatering what exactly are we (the global we that is) trying to do? The answer to that is quite simple and it is to waste the right amount of water depending upon the thickness of sludge that we want and can achieve at an economic level that the process can cope with. That seems a bit long-winded but let me explain with sludge thickening.
When we thicken sludge we take it to a level of thickness that the downstream pumps (or conveyors) can cope with. The sludge will then go on for transport somewhere else or if at the sludge treatment facility then it will be treated somewhere else. Now if it is transported and it is too thick then the sludge can not be withdrawn by a tanker and there are transport problems. If it is treated onsite then any pumping will also cope without specialist pumps. Coupled with the fact as to what is done to that sludge to get it to that state then this can also can cause problems, i.e. usually too much polymer that is (a) expensive and (b) can cause problems with foaming and (c) can actually cause isostatic repulsion which actually cause the sludge to be thinner than if less polymer were added.
So what do we actually do when we thicken or dewater a sludge and what are the key factors? They are simply put in the box diagram below:
If you breakdown the different elements of the process that need to be controlled you simply get what you need to know:
- Sludge thickness and flow rate into the process
- Polymer flow into the process
- Sludge thickness and flow rate out of the process
- Volume (and perhaps quality of sludge liquors) out of the process.
What does this require instrumentally? Again quite simply
- Flow and dry solids measurement into the process
- Polymer flow rate (assuming the strength of the polymer is kept the same)
- Flow and dry solids measurement out of the process
The sludge liquors can be calculated from the amount (volume/mass) of sludge in – the amount out. The alternative is to monitor the liquors and calculate the sludge out. This basically tells the operator how much sludge they are producing and the mass balance through the system. That is the basics…….
Now onto something a little bit more complex and the only addition is a PLC and some control systems. This is all based on the fact that some of the variables such as polymer dosing rates have been looked at and the thickening/dewatering systems have been set up by one of the specialist suppliers. The problems that are typically caused in sludge thickening and dewatering is the mass of the incoming sludge, these systems like things nice and consistent as normally the polymer dosing rate has to be manually changed, the question is why?
The answer is that there is no reason to do so. There are commercial control systems out there that will manage your sludge thickening/dewatering process but I think the best I have seen was developed by two very intelligent people at Yorkshire Water as it not only managed the polymer dosing system but it is also monitored the polymer dosing rates in pounds and pence. The basic system was a little difficult to commission but once the principles were done it works very well. The principle is of course to monitor the mass of the incoming sludge and alter the pump dosing rates to work on mass based thickening. This way the amount of polymer dosed per tonne of dry solids were controlled and so where the costs. The system did not of course stop there as there was a feedback loop that controlled the sludge thickness, if it was too high and the sludge was coming out too thick the polymer dosing rate was decreased too thin the sludge and vice a versa if the sludge was too thin.
What are the disadvantages of this system? Mainly the instrument maintenance….dry solids meters do not react well to thick sludges passing through them as the clean can be (too put it lightly) a little bit intensive but if maintained the system can work well and the quality of the liquors tend to be superb.
In next week’s blog Oliver will attempt to go into some of the more simple control systems around sludge digestion