Up to 80 percent savings: HUBER Screw Press Q-PRESS® minimises electricity costs for sludge dewatering

Screw presses have been proving their worth for many years at wastewater treatment plants with population equivalents ≤ 100,000 mainly because the simple operating principle offers advantages compared to other continuously operating dewatering units such as decanter centrifuges:

• The use of the sludge feed pressure (0 – 300 mbar) as a control variable enables automatic adjustment of the machine and flocculant settings to fluctuations in the sludge consistency. Time-consuming readjustments by the operator are thus reduced to a minimum, and the dewatering system can typically be operated unattended overnight or over the weekend.
• With continuous operation of the sludge dewatering unit, the back-loading of the treatment plant by the filtrate water is evened out. Additional buffers to protect the biological stage from back-loading peaks can be omitted.
• he slow speed of the screw shaft (< 1 rpm) enables low-wear operation and thus reduces maintenance times and costs for spare parts.

Dewatering degree and flocculant consumption determine operating costs

However, the operating costs and thus the economic efficiency of sludge dewatering are largely determined by the degree of dewatering and the flocculant consumption. These parameters are typically determined in advance by on-site tests on a real scale or in the laboratory, evaluated in the tender process and thus used to award the contract. The increasing sales figures of the HUBER Screw Press Q-PRESS® prove its competitiveness here.

Rising energy prices, energy self-sufficiency and security of supply

Although the energy costs of sludge dewatering have so far played only a minor role in economic efficiency, a detailed consideration may make sense under the aspects of rising energy prices, security of supply or even energy self-sufficiency.

Electricity consumption of sludge dewatering

The electricity consumption of a sludge dewatering plant typically consists of the individual consumptions of feed pumps, conveying units, the flocculant preparation unit and the dewatering unit itself. While the peripheral plant components of a sludge dewatering system can typically be assumed to have a specific electricity consumption of approx. 15 kWh/tDR across all manufacturers, there are significant differences for the dewatering units.

Power consumption of a HUBER Screw Press Q-PRESS® demo unit

The diagram shows an exemplary evaluation of the power consumption of individual consumers of a demonstration unit of the HUBER Q-PRESS® at different wastewater treatment plants and under different load conditions. The specific power consumption of the peripheral plant components (without screw press) is approx. 15 kWh/tTR. Experience shows that this proportion is largely independent of the dewatering technology used.

Only 30 per cent of the total electricity consumption of sludge dewatering

The diagram shows that the screw press requires about 5 kWh/tDR and thus accounts for only 30% of the total electricity consumption of the sludge dewatering process. Differences in the specific energy requirements between the test locations are due to different solids loads and load conditions.

Compared to decanter centrifuges: screw press saves around 80 per cent energy

Using a modern decanter centrifuge results in a specific power consumption of about 40 kWh/tDR for the dewatering unit alone. The use of a screw press can therefore save approx. 80% energy compared to a decanter centrifuge.

The following example calculation, based on a sludge quantity of 1,000 t dry matter (approx. 50,000 PE), illustrates the financial savings potential: Assuming an electricity price of 0.26 EUR/kWh, the annual electricity costs for the decanter centrifuge are 10,400 euros, for the screw press only 1,300 euros.

A similar saving in operating costs of around 9,000 euros per year would result, for example, from:

• a permanent increase in the degree of dewatering by about 0.5 percentage points, assuming disposal costs of 100 EUR/tDR
• an average reduction in polymer demand of 3 kg/tDR at polymer costs of 3 EUR/kg

The low energy consumption of a screw press can thus influence the economic comparison with decanter centrifuges and significantly reduce the electricity consumption for sludge dewatering.