Best-in-Class Mechanical Dewatering Methods
Depending on what industry you're in the term "dewatering" can mean different things. Construction site dewatering typically refers to the management of water on a site, whether it be pond water or ground water to allow construction activities to continue uninhibited. In this context dewatering typically focuses on the removal of water from the site through pumping and its corresponding treatment and discharge.
In contrast wastewater dewatering typically focuses on slurry liquid solid separation to reduce waste volumes and provide waste management options. The wastewater dewatering process is one step in the operation of a wastewater treatment system. Optimization of wastewater dewatering then typically involves maximizing throughput, minimizing costs and maximizing separation.
Optimizing the Dewatering Process
Mechanical dewatering is a common method to reduce and manage liquid waste streams. Factors to consider in choosing and optimizing your dewatering process include:
- Polymer dosage
- Equipment maintenance
- Ease of operation
- Sludge Feed
- Effluent characteristic requirements
- Volumes to be processed
- End use
Best-in-Class Mechanical Dewatering Methods
The number of dewatering methods are as varied as the number of liquid waste streams they treat. As one component of a treatment system, variation in implementation is limitless. The three most common methods of wastewater dewatering are centrifuge, belt press and geo-textile bags.
The centrifuge spins the polymerized sludge feed at a high speed to separate the solids and liquid. They have a high power requirement and polymer dosage rate. They benefit from a smaller foot print, low maintenance and ease of operation when used with a consistent sludge feed. Centrifuge units are common place in many large wastewater treatment facilities and achieve solids concentrations anywhere from 18-30%.
Belt presses come in a variety of designs but all work on the principal of squeezing the polymerized sludge feed to separate the water from the solids. The belt is permeable to water but not solids allowing their separation. They benefit from lower power requirements and dosage rates. They have higher maintenance and require more operational oversight. They work well with consistent sludge feed. Belt presses are common in many small to mid-size wastewater treatment facility and achieve solids concentrations anywhere from 18-28%.
Geo-textile bags provide a unique dewatering option. They work by trapping the solids inside the bag and allowing the water to permeate through the geo-textile. This method typically requires a large footprint to setup, although some unit have been setup in a standard waste bin. The bags allow for large volumes to be dewatered quickly and have been used in the past during lagoon clean outs. The bags are a one-time use and once full are disposed of increasing the cost of the operation for larger steady sludge feed flows. They are able to handle fluctuations in solids content rather well and benefit from lower polymer dosage rates. They also can provide storage for the captured solids (which will continue to desiccate) while awaiting final disposal or reuse. Because the dewatered sludge can be left to desiccate for any amount of time this method can achieve solids concentrations anywhere from 18-55%.
Rotary drum dewatering units while used throughout the world are a newer entrant into the dewatering arena and perhaps serve a niche in the market. Wessuc first started using rotary units in 2010 when we used a vacuum unit. Since then we have switched to a rotary fill unit which works in much the same principal as a geo-textile bag. The rotary drum is filled to capacity while the liquid permeates through the outer shell. Once the unit is filled, the material is rotated for a number of hours to expedite the drying process. This method benefits from low polymer usage, low energy requirements and simple operation. The units can handle high solids fluctuations in the sludge feed and produce a high quality effluent (less than 100 mg/L TSS). The higher solids found in a digester clean out project result in dewatered solids concentration of 25-50%.
3 Important Best Practices in Detwatering
1. Testing and Evaluation
Polymer dosage rates will vary greatly depending on the dewatering method being used and the liquid waste characteristics. Liquids with higher solids concentrations will require higher dosage rates. Different dewatering methods will require different dosage rates to achieve equal solids dryness results which related directly to the overall dewatering cost. Polymer cost are often more than 50% of the input cost required for a treatment system and as such require testing and evaluation to determine the optimal type and dosage.
2. Automating Methods
Ease of operation is a key factor in the overall cost of the equipment. The more automated the equipment the less man hours required to supervise, trouble shoot, restart, and clean up the equipment. Therefore, the more a method can be automated, the more time operators can spend optimizing other areas of the wastewater treatment process.
3. Achieving High Quality Effluent
Effluent quality required from the dewatering operation will directly impact the available options. High quality effluent while achievable often requires a slower process with solids higher capture rates. For most wastewater treatment plants, the effluent quality is not the determining factor as it is typically returned to the plant for reprocessing and not directly discharged to the environment.
Wessuc has experience completing projects with each of these best practices and has been working to improve the dewatering process in Canada for over 10 years to achieve efficiency and reduce cost and environmental impact. Contact us for more information on dewatering Canada.