What is the best way to size a dewatering machine for WWTPʼs best results?

Sludge dewatering at wastewater treatment plants is a complex discipline that is not necessarily complicated but is governed by precise rules. If you follow certain dewatering principles, you can achieve excellent results, and the dry solids content of the output can be as high as a respectable 22%. Selecting a suitable dewatering machine and continuous work with the sludge will lead you to the desired goal.

Preparing sludge for dewatering

It is not only the dewatering device and its parameters that determine how effective sludge dewatering will be. Even the most powerful machine still works with the material you send to it from the wastewater treatment plant (WWTP). Sludge quality matters, something that many WWTP operators still need to realize. We've already written about how to care for sludge so that the final solids content is as high as possible.

Dewatering machine: the alpha and omega of the whole process

The choice of appropriate dewatering technology plays an important role. Do you know what you are choosing from? We have compiled the most common types of 21st-century dewatering systems, their advantages, and their disadvantages into a clear comparison. An efficient and investment-saving device with minimal operating noise is our screw press.

A screw press, but how big? 

Choosing the right size is almost as crucial as choosing the machine itself. Not all users of wastewater treatment plants, or even the designers who design WWTPs, know what parameter to grasp when designing a technology for sludge dewatering in wastewater treatment plants. Both pumps and blowers only need to know two parameters - flow and pressure. So what about the dewatering machine?

We often find that the designer wants to design a dewatering machine, and the brief is something like this, "We have a flow rate of 4 cubic meters per hour at the WWTP; oversize the machine we can put in."

But my question immediately follows, "What percentage of dry solids do you have in the sludge?" In practice, if you have 2% dry solids in the influent, you count on 20 kg of dry sludge per 1 cubic meter of water.

Dewatering machine: 6 sizes for everyone

Sometimes it can be challenging to make sense of the numbers, but our markings have a clear logic. We sell six types: 131, 201, 301, 401, 402, 403. The first two numbers indicate the diameter of the screw, which indicates the power for that particular dry matter (not the flow rate), and the last number indicates how many screws are in parallel in the machine. One example says it all: 302 = the screw diameter is 300 mm and contains two screws in parallel. Each screw dewatering screw press has a defined mean value in the parameters for how many kg of dry matter it processes per hour. The machine should consistently achieve this value even in the case of "non-ideal" sludge. However, when the sludge is sufficiently stabilized, the machine can achieve up to four times the output.

How to calculate in practice?

If the designer states that the WWTP has a flow rate of 4 cubic meters per hour and 2% dry solids, that means 20 kg per cubic meter, for a total of 80 kg of dry solids per hour. Our task is to select a machine that will reliably give us at least this 80 kg per hour, a machine with the designation 302. The 302 will process up to 120 kg (and the 301 only 60 kg, a notch lower).

But how to design how much sludge the WWTP will produce per day? Statistical measurements and design experience suggest 50 g of solids per person daily. If we have a municipality of 1 000 EO (equivalent inhabitants), this means 1 000 × 50 g, which is 50 000 g, that is 50 kg of solids per day. And it is also a known figure that 100 l of water per person per day flows into the treatment plant (sewage, washing machine, dishwasher).

The designer holds the primary decision-making power

It is essential to size the wastewater treatment plant project correctly. Just right and with a contingency for possible growth. However, it certainly does not make sense to include a machine twice as big as is needed because it will put an unnecessary strain on the budget, and, more importantly, the machine will not be in efficient circulation. The ideal situation is when the machine runs two to three days a week. Overfunding, generous budgets, and fear of underestimation of size often influence the designer. However, designers should also consider the risks of oversizing.

Sludge dewatering at municipal or industrial wastewater treatment plants doesn't have to be rocket science if you know how to do it. Or if you take advice from the experts.

We will be delighted to help you choose the correct size dewatering machine. Contact us.