O. Miasoiedov, K. Jarmołowska
Abstract. Due to the necessity for resource consumption reduction, wastewater treatment plants are constantly striving to improve existing treatment technologies . This question is often related to reductions in chemical reagents consumption; particularly for the sludge dewatering process. In order to analyse the corresponding cost savings, flocculant doses used on a belt filter press as well as on a multi-disc screw press were compared on pilot studies carried out on a municipal wastewater treatment plant. The results showed several times less the consumption of resources needed to prepare the flocculant solution and a reduction in the monthly resource purchase expense. This information will be useful for estimating operational costs for a sewage treatment plant.
Keywords:wastewater treatment plant, sludge dewatering, flocculant solution, flocculant dose, belt filter press, multi disc screw press, resources consumption, operating costs, municipal sludge.
Introduction. Currently, rising utility prices and the development of global environmental protection activities will compel wastewater treatment plants to look for the most economical and highly automated technologies which will be able to achieve high process efficiency. Sludge dewatering is a critical process for every municipal wastewater treatment plant and is a very dynamic process which can be changed and adjusted in a wide range of technological parameters. Operating costs savings achieved by sludge dewatering technology and equipment modernization may include the reduction of dewatered sludge volume as well as reductions in electricity, water and reagent consumption such as flocculant. The flocculant dose may differ several times depending on specific sludge composition , which could significantly influence overall expenses.
Object and conducted study description. In order to establish the significance of flocculant dose reduction on operating costs and reagent consumption at a wastewater treatment plant where a belt filter press is currently installed, the data obtained during the pilot tests of both the belt filter press and multi disc screw press were compared. In order to carry out reliable calculations of the flocculant doses consumption, a treatment plant has provided the most up-to-date data from the last months operations.
The wastewater treatment plant serves approximately 15,000 residents as well as several small industrial plants (fruit and meat processing) in the region. The wastewater treatment technology includes mechanical treatment (as well as fine treatment), biological treatment (phosphorus removal, nitrification and denitrification), gravity thickening and sludge dewatering.
Currently, the aerobically stabilized surplus activated sludge dewatering process takes place on belt filter presses with flocculant treatment. Due to the need for increasing the dewatering process efficiency and obtaining costs reductions, a decision was made to improve the existing dewatering technology. For that reason, pilot tests of a multi-disc screw press were carried out. The results obtained were taken for reagent consumption calculations and comparisons with current results. The studies showed that the multi-disc screw press produces high dewatering efficiency and good results with this kind of sludge and can successfully replace the belt filter press with a dewatering process performance improvement (table 1) at municipal wastewater treatment plants.
Table 1. Pilot studies results
|Equipment||Dry solids (DS) content in sludge||DS content in dewatered cake||Total Suspended solids (TSS) in filtrate||Flocculant dosage|
|Belt filter press||0.85 %||13.3 %||800 mg/l||800 mg/l|
|Multi-disc screw press||0,86 %||15,0 %||< 50 mg/l||2,1 kg/tDS|
Results. In order to confirm the operating costs savings resulting from the flocculant consumption reduction, the data from a wastewater treatment plant for December 2017 were analyzed. For the period, 4,642 м3 of sludge was generated which corresponds to 1,32 TDS/d. A flocculant price of 3 EUR/kg was taken for the calculations (table 2).
Table 2. Flocculant consumption and related operating costs calculation comparison
|Parameter||Average flocculant dose||Daily flocculant consumption||Monthly cost (30 days)|
|Belt filter press||7,0 kg/tDS||9,24 kg/d||831,60 EUR|
|Multi-disc screw press||2,1 kg/tDS||2,77 kg/d||249,30 EUR|
Conclusions. Flocculant doses which were used for calculations were obtained during pilot studies for this test and provide better dewatering results (table 1). Actual dosage is less than average for other municipal sludge dewatering plants [3, 4]. Apparently, this occurs due to the specific sludge composition (mixture of industrial wastewater).
The Multi-disc screw press in comparison with the belt filter press allows a savings of 582,3 EUR monthly, which corresponds to approx. 70% of the wastewater treatment plants reagent expenditures . It means that the relative costs savings of 14,70 EUR/tDS (4,9 kg of flocculant per TDS) could be applied on any other similar plant.
Additionally, belt filter press requires 4,62 m3 of water for flocculant dissolving (0,2 % solution). Due to lower flocculant dosage, multi-disc screw press requires only 1,39 m3. This allows a savings of approximately 70% of water consumption as well, which corresponds to 96,9 m3 monthly just for the flocculant dissolving process.
Additionally, belt filter press requires 4,62 m3 of water for flocculant dissolving (0,2% solution). Due to lower flocculant dosage, multi-disc screw press requires only 1,39 m3. This allows a savings of approximately 70% of water consumption as well, which corresponds to 96,9 m3 monthly just for the flocculant dissolving process..
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