For many decades, Dissolved Air Flotation (DAF) systems have been one of the most popular means of primary treatment of industrial wastewater. The rates of solids capture by DAF systems are generally excellent being rarely less than 95 per cent when an appropriate chemical program is applied and typically produces a treated wastewater stream that is suitable for discharge to sewer or to a secondary treatment process.

When applying a DAF system to any specific industrial wastewater project, the design is based on a number of factors, including the flow rate, surface loading rate and the air-to-solids ratio. In cases with low to average suspended solids concentration in the wastewater, the dissolved air recycle rate can vary from 15 to 50 per cent of the feed flow.

When handling very high loads of solids, the amount of dissolved air required to achieve a suitable air-to-solids ratio can increase to several times greater than the feed flow. This has a considerable impact on the hydraulic load on the DAF so, to ensure an appropriate surface-loading rate, a much larger DAF system is required.

For example, a DAF system installed in an application where the average suspended solids concentration is 10,000 mg/L will require a surface area of at least two to three times greater than that required for a waste stream containing only 1000 – 2000 mg/L.

Sludge generated by a DAF system rarely exceeds ten per cent dry solids content.

Sludge generated by a DAF system rarely exceeds ten per cent dry solids content.

The solids removed by the DAF process will always contain a reasonable amount of water. Depending on the type or nature of the wastewater, the chemical program applied and the efficiency of the DAF sludge removal mechanism, the dry solids concentration in DAF sludge can vary from three to ten per cent – or perhaps higher in some isolated cases.

Because of this, DAF sludge is often further processed in a dewatering system such as a screw press which will significantly reduce the water content in the sludge via compression to generate a spadable sludge with up to a ten-fold decrease in sludge volume.

So, a wastewater treatment plant will often have both a wastewater treatment process and sludge dewatering process. It is worth noting that a sludge dewatering system will typically cost a similar amount to the DAF system and can take up a similar amount of space.

A one device solution

A screw press is suitable for dewatering DAF sludge which usually contains somewhere between three and ten per cent dry solids. When designing, or selecting a screw press for a dewatering project, consideration is given to the hydraulic load or flow rate of the sludge to be dewatered and its solids content. For example, a HUBER Q440 QPRESS unit would be selected to dewater three kL of sludge per hour containing ten per cent dry solids.

If one looks more closely at how the screw press works, one can deduce that, providing the screw press is designed and sized accordingly, there is no reason why a wastewater stream with a medium to high solids load cannot be pumped directly into a screw press. By doing this, a spadable sludge is the by-product of the complete treatment process and the treated water or filtrate will have a residual solid load akin to a DAF.

How does a screw press work?

After many decades, the DAF system is still the optimum treatment process for many industrial applications.

After many decades, the DAF system is still the optimum treatment process for many industrial applications.

It is the first part of the screw press operation that is the most important aspect when selecting a screw press as a complete wastewater treatment process. In the initial low pressure zone, free water flows through the internal filter so if the low pressure zone (or more specifically the internal filter surface area) has the capacity to allow all the free water in the flocculated sludge to flow through the filter at the rate it enters the press and still capture the solids, then the relative percentages of free water compared to the amount of solids should not matter.

Given that screw presses are currently operational and dewatering waste activated sludge containing as little as one per cent solids or even less, there is no apparent reason why this theory cannot be applied to dewatering industrial wastewater that contains a similar solids concentration.

Hydroflux has been conducting trials on screw presses as a combined solids-separation and dewatering device and has successfully implemented this process in industrial applications where typically a DAF system and a screw press would have been installed. The Hydroflux installation has resulted in significantly lower capital costs and space savings.

The use of a screw press in this manner is not always the answer. The characteristics of the wastewater need to be very consistent and it needs to flocculate well with relatively high suspended-solids concentrations and flow rates at the lower end of the industrial spectrum. The type of screw press is also important. Units with large initial low pressure zones and purpose selected wedge wire sizes are critical as is engaging specialists with real hands on knowledge of the industrial wastewater process and the application of screw presses.

This partner content is brought to you by Hydroflux. For more information, visit 


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