Generic Sizing Of Sludge (Biosolids) Dewatering Plant

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Introduction

Sludge Dewatering and thickening is one of the most common processes in Water and Wastewater treatment plants. Belt presses, gravity tables and centrifuges are the usual equipment items used. In the case of belt presses and gravity tables, the simplicity of the equipment can lull operators and maintenance staff into a slow process of allowing the machines and settings to deteriorate.

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Dewatering Equipment Capacities

Biosolids dewatering equipment have two capacities. These are the Hydraulic (Volumetric) flow rate capability, and the Dry Solids throughput capacity.

For every dewatering machine, one of these parameters is going to be the limiting factor on a particular application.

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• Centrifuge type machines have to accelerate the full incoming flow, and therefore the liquid mass of the volumetric flow can be a limiting factor.

• For filtration type devices such as belt filter presses and screw presses, for typical feed rates above 1% solids, the dry solids throughput capacity is usually the limiting factor.
   

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Dry Solids Throughput Capacity

For selecting and specifying a sludge dewatering machine, it is therefore important to nominate the Dry Solids design duty in kilograms/hour throughput, in addition to the Volumetric capacity.

The dewatering characteristics of the biosolids will have a major influence on the capacity of the dewatering device. It is therefore very important in specifying a dewatering machine, to define the characteristics of the sludge. Unfortunately, the dewatering characteristics of the sludge can be influenced by a very large number of parameters, including the treatment plant influent, type of process, duration of and extent of the treatment, and many other factors.

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Apart from defining the basic treatment process, such as nominating anaerobic digestion with a sludge age of (x) days, the most useful dewatering characteristic is the ash or volatile content of the dry solids. This will provide some guidance to the equipment supplier as to the basic nature of the sludge.

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Determining The Duty Kilograms/hr Throughput Rate.

Stepping back from the details of the process plant, the standard rule of thumb is that most treatment processes produce around 50g of dry solids per EP (Equivalent Persons) treated by the plant. Where the treatment plant influent comprises a mixture of domestic and industrial waste water, it is necessary to convert the industrial load to an EP. Hence for an example 10,000 EP plant, the rough estimation is a solids load of 500 kg/day.

The next step in sizing the dewatering equipment is to determine the operating hours during a working week. Biosolids dewatering requires the use of polymer flocculants and adjustment of the dosage rates and equipment parameters to suit the combination of flocculant and biosolids characteristics. Unfortunately with the vast number of variables involved, the most reliable method of control is a steady feed and operator visual input. Hence the preference is to operate the dewatering during normal working hours, of say seven hours/day, five days/week. For a typical treatment plant with only one dewatering train, this leaves additional days/week and hours/day for coping with process or maintenance upsets.

For our 10,000 EP example, we would then have a design duty of 500kg/7 hours/day times 7/5 days/week = 100 kg/hr design duty.

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Volumetric Throughput Capacity

The next step is to determine the required volumetric capacity. This is usually dictated by the upstream process.

 

For the 10,000 EP example, an anaerobic digester might be designed to operate with a 3% solids discharge. In this case the lowest required volumetric capacity would be: 0.1 m³/0.03 per unit/hr = 3.33 m³/hr. (SG of biological sludge is close to 1.)

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If however, the digester is operating outside its design duty, the solids content could drop to 1.5%. In this case we would still need to maintain the dewatering solids throughput, and therefore the volumetric design capacity would need to be doubled to 6.66 m³ per hour.

 

Specification Duty

The final specification for the equipment for our 10,000 EP example would therefore be a solids throughput requirement of 100 kg/hr and a minimum volumetric capacity (rounded) of 7m³/hr of anaerobic digested sludge with a volatile content of (x) percent.

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