Schwing Bioset Screw Press

I’m a big fan of things that are easy to use and just work. That’s exactly what I’ve noticed with the Schwing Bioset screw presses. With minimal settings, you can go to the HMI panel and get them started with the push of a button. We are a smaller plant where I require the operators to be able to do other things, we couldn’t afford to have operators just exclusively pressing all day. The Schwing presses do a very good job of allowing the operator to do some pre-checks, start it up, and walk away to do other things. There are a lot of safety features built in that will automatically shut the press down. We can run these with very minimal oversight and adjust throughout the day allowing us to accomplish other tasks.

The performance is good as well. We can press up to 950 dry tons per hour, per press, but we normally do not have to run these at full speed to accomplish our waste goals for the week. We have found running the presses slower is better when it comes to getting a drier cake. I feel that this is where these presses really are a work horse. If we monitor our % solids of the incoming biosolids to the presses, we can easily make adjusts to the polymer feed rate and get results of upwards of 25-30 percent solids on the cake. Most of the time we run closer to 20 percent solids on the cake, but I’ve seen it go above thirty when we are really dialed in. Our goal is to get the driest biosolids cake in the dumpster as possible and these presses are definitely capable of doing that.

The various sensors and safety mechanisms built into the press can be a bit frustrating sometimes. It isn’t that I am not happy that they have safeguards, it’s just the way in which they work. On each floc tank (where the biosolids are mixed with the polymer) there are PSI gauges. These PSI gauges essentially control the throughput of the screw press. For obvious reasons, if this PSI gauge gets too high it will shut the system down to prevent any catastrophic damage. The problem with this is the default logic is really slow when it comes to throttling the speed of the pump up and down. Sometimes you’ll get thicker sludge going through the system and the PSI will increase faster than the pump can throttle down, and it’ll shut the system down. One solution is to increase the screw speed and that’ll usually solve it, or you can dial the system in manually (not recommended per Schwing). 

Another problem with encountered are with the laser sensors that monitor the level of cake in the dumpsters. These lasers shoot directly down into the dumpster and relay it back the PLC. Based on what percentage you input, it’ll shut the door on the front section of the auger system, forcing it to the back of the dumpster. Once the back portion of the dumpster fills to a set percentage, it’ll shut the system down. You can change this at the HMI at any time. They are very accurate most of the time. One issue we encountered was that this kept “bouncing” going from 5% to 95% and back down. This would shut the press off prematurely. I’ve worked with Schwing on this and never got a solid answer on why they do this other than there must be some sort of interference. We replaced a few of sensors themselves and a wiring harness, and that seemed to have solved it. Except for press No.1, which not only would bounce but eventually stayed above 100 percent and never allowed us to run the press in auto as it was reading the dumpster as full. We worked with Schwing on this as well and never came to a satisfactory solution. So we unplugged that sensor and have to manually shut the press off each day. One issue we also encountered is that there is no safety or limit on if the chute plugs on the end of the screw. It’ll literally blow the doors off the hopper where the cake drops into at the end of the screw as seen below.

Maintenance on these systems is pretty standard. Some greasing and oil changes in the gearboxes. The biggest maintenance item on these presses is the lip seal that is inside the cage and screw. This is quite involved, and we will likely outsource the first attempt of this to Schwing. We haven’t had to do this yet, but we will soon, so it’ll be interesting to see how difficult and what the cost will ultimately be. Other than that, we haven’t had any issues other than with the sensors. The units we had came with the sensors already affixed to the bottom of the auger and were very difficult to remove due to the clearance between the sensor and the end of the auger casing.

Customer service with Schwing has been pretty good. They’ve always been responsive, quick to offer services, and assist in troubleshooting. However, getting them to stand by some of the parts on the equipment required quite a bit of end-user hassle. If something is clearly not working and we have done all the recommended troubleshooting, I expect it to be replaced if it is within the warranty period. We had a bit of push back when it came to replacing the level sensors for the dumpsters that clearly were not working from start up. Ultimately, they did end up sending us new sensors so I can’t complain too much. Lastly, anything you have them do directly will likely be expensive. This isn’t exclusive to Schwing. However, sending a technician out from any company is going to be quite pricey, so keep that on mind anytime you think of contacting them directly for service. Overall, I am happy with their customer service but realize you may be doing a lot of the leg work.

If you’re looking at biosolids handling units, I’d certainly give the Schwing screw press a look. They are workhorses that are nearly fully automated and allow you to get rolling with the push of a button. They do not quite have the output of a centrifuge but require a lot of loss coddling and adjustments (at least in my experience). In comparison to other brands like FKC, I don’t know how they match up as I have never used FKC. I can only compare these to Andritz belt presses and Andritz centrifuges as those are what I have experience managing. In the two years that my crew and I have operated these presses we have had minimal down time.