Profitably managing waste. It’s the goal of every municipal wastewater treatment plant. Critical to achieving this goal is the minimizing of waste weight and volume. To do this, WWTP’s deploy dewatering systems to separate out the liquid portion of the waste, decreasing disposal costs in the process.
Traditional dewatering solutions like Belt Presses reduce liquid content down to approximately 20%, however these solutions present their own set of challenges, including:
And even with oversight and maintenance, many Belt Presses don’t last more than 10 years without a complete overhaul. There is a better and simpler way to dewater municipal waste!
The Rotary Press is an emerging dewatering solution that simplifies the dewatering process. Used to separate cake from filtrate, the Rotary Press is an alternative to solutions like belt presses, centrifuges, and screw presses.
Intake is piped in through the pump, which drives the sludge at a measured rate. From there, an Emulsion Polymer System dilutes polymer with water and injects this mixture into the sludge. The sludge is then processed through durable filter screens where the filtrate is pressed out and expelled from the press. After sufficient pressure is built, cake is discharged through the chutes.
The resulting cake weighs a fraction of non-processed sludge and with a dryness of 22% total TS, (aerobically digested sludge) often far less than sludge processed by many traditional dewatering technologies, yielding greater savings on waste disposal costs.
As technology advances, dewatering systems are becoming increasingly efficient in delivering dry cake, reducing downtime due to maintenance, and lowering operating costs. Continue reading for a full comparison of the Belt Press and Rotary Press.
To initially prepare the Belt Press for use, operators generally configure the equipment, a process taking up to 30 minutes. First, the sludge pump needs to be primed. Next, operators must prime the polymer to make sure it’s delivering an adequate flock size. Once this is done, the belt speed, auger speed, and wash water are adjusted. Of particular note here is the degree of wash water consumption. Wash water runs continually as the belt press is in use—it's the first thing turned on during the initial startup and it's the last thing turned off during shutdown, consuming roughly 40-100 gpm depending on the size of the belt press.
Once up and running, the Belt Press typically requires frequent monitoring throughout the day, generally every 30-60 minutes, to ensure that it's running properly and operating within the set parameters. Maintaining appropriate sludge distribution is particularly important, as uneven distribution negatively affects belt tracking. Maintaining belt tracking is critical and correcting improper belt tracking is a long and arduous task that causes significant downtime.
Another source of difficulties includes the potential for plugging of the polymer line. When this occurs, the press will continue to run without dewatering the sludge, which can result in sludge pouring over the top of the press. If plugged, after stopping the Belt Press, the system must be cleared and all parameters reset, a process that takes as much as 30 minutes.
Size is another major consideration. Belt Presses tend to be large, with even smaller units measuring around 20’ x 7’ and are often positioned on pillars, further increasing their bulk. Further space is taken by feed pumps, polymer systems, air compressors and hydraulic pumps. Due to space constraints, these systems are sometimes positioned outside, which can present issues with noise generated by constantly operating drums, pumps, and motors in addition to the continuous running of wash-water.
The compact plug-and-play Rotary Press is delivered on a skid and requires only a fraction of the room required by most Belt Presses, diminishing expenses associated with housing the unit. A simple startup process consists of turning the air compressors and the power on. Nearly all operations are controlled from the touchscreen, including setting the sludge pump speed, the gate pressure level, and inputting the desired wash cycles. Once the chemistry is adjusted, the daily setup process takes approximately 15 to 20 minutes.
After the desired parameters are set and the Press is functioning, the system is capable of operating unattended. The polymer rate, pump speed, and gate pressure consistently perform according to set parameters, freeing the operator to attend to other work. Should a problem occur, for example due to pressure values exceeding set limits, an alarm will alert the operator, and the system will automatically halt the dewatering process until the operator has opportunity to resolve the issue.
One of the most welcomed aspects of the Rotary Press among operators is its contribution to water conservation. The Rotary Press uses wash-water only during brief and intermittent wash cycles. Standard operating procedures include a quick wash cycle during startup and shutdown. Operators can set additional wash cycles as needed, managing the duration of those cycles from the touchscreen.
Belt Presses deliver fairly dry cake of up to 22% (aerobically digested). Many factors influence this, including the thickness of the sludge, amount of polymer, belt tension, and speed. If the polymer is not adjusted properly, it will result in a much wetter cake. It’s also critical to clean the belts frequently to prevent sludge overflow.
The Rotary Press consistently produces among the driest cake of any dewatering solution outside of plate style filter presses with a modest amount of polymer, lowering waste disposal costs and minimizing polymer consumption.
The Belt Press typically requires 30-45 minutes of cleaning daily to maintain normal operations. Because there are many places on the Belt Press where solids frequently accumulate, daily rinsing and washing of the belt, augers, and all the slides where sludge is going is often necessary. Weekly greasing is also generally recommended to keep the belt running smoothly on its track.
Regular maintenance is typically needed due to the constant friction of moving parts. One of the most common issues with the Belt Press is caused by errors with the sensors located on both sides of the belt system. If the sensors aren't working properly, they won’t alert operators to the belt jumping off the track. When this happens, the Belt Press typically must be shut down so operators can reverse the belt for realignment, a process that can take up to an hour. If any parts break, a repair team is required to assess the damage and fix the problem. This could result in a shutdown of anywhere from a few hours to a few days.
Cleaning is automated and seamless with the Rotary Press. Operators can set and change wash cycle parameters to control how often wash cycles occur and how long they last. Pre-shut down automated wash cycles take about 12 minutes and require no oversight from the operator.
All key wear parts are easily accessible on the Rotary Press. Pumps, motors, and their check valves are exposed and easy to get to.
The simplicity of the Rotary Press design results in reduced maintenance requirements. Simply stated, there is not much that can fail on it compared to Belt Presses and other dewatering solutions. The Rotary Press has no belts, bearings, or high RPM scrolls. Instead, a slowly rotating screen (less than 1 RPM) facilitates the filtration process, diminishing downtime and reducing maintenance costs.
The Rotary Press is operator-safe and user-friendly. There are no fast-moving belts and rollers to be concerned about. The sludge is contained within the system, and won’t spill out and cover the platform. The relatively small, self-contained units require little monitoring. There is no scaling of ladders or potentially tripping on sludge covered platforms. From a safety and ease of use standpoint, the Rotary Press is an ideal solution.
ROTARY PRESS |
BELT PRESS |
|
| Wash-Water Consumption (based on 8-hour shift)* |
Water Consumption: circa 3,200 gallons | Water Consumption: circa 57,600 gallons |
| Electrical Consumption | 0.75 - 13 hp | 15 hp |
| Hydraulic throughput (gpm) | 5 - 260 | 20 - 150/m |
| Cake Dryness (% TS) | 15% - 22% | 12% -20% |
| Operations | Automated system alerts operator and halts production in the event of a problem | Monitoring required to prevent spills and downtime |
| Cleaning | Automated, 12-minute wash cycles controlled by operators | Manual, 30-45 minutes of cleaning by operators after daily use |
| Maintenance | Low, infrequent, simple. Completed by one person. Screens can last over 10 years. | High maintenance belts, rollers, bearings, sensors, seals, wash nozzles, and doctor blades. Building maintenance due to corrosive gases |
| Safety | Very few hazards, equipment is self-contained | Potential hazards at pinch points |
| Startup Time | 15 - 20 minutes, everything configured electronically | 20 - 30 minutes, everything configured manually |
| Footprint | Small Footprint: 24" units = 6' x 5' 48" units = 9' x 8' |
Large Footprint (typical): 1M units = 18' x 7' 2M = 22' x 11' |
*Based on recommended wash cycles for a 48” D Rotary Press, and a 1.5 M Belt Filter Press
Looking to reduce operating and maintenance costs, improve efficiencies, and create an effective dewatering process? Contact your Evoqua sales rep or email dewatering@evoqua.com to receive a complimentary sludge analysis and an evaluation of the best dewatering solution for your application.
