Central Boiler Maxim and burning sludge pellets

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New Member
Jan 13, 2023
South central Wisconsin
Hello all,

First time posting on this forum, I’ve been on heating help for a little while and have been lurking on here. I’ve been watching for a good used outdoor wood boiler for a while, but have changed my tune towards a pellet boiler to heat my old Wisconsin farm house.

I work at the local waste water plant where we dry the sludge, and are looking into getting a pellet mill to pelletize the final product. We give away all the sludge, so I could have an unlimited supply of pellets. I also farm, so corn is always an easy option when the price is right.

Now I see for sale locally is a pair of central boiler maxim 255pe corn/pellet boilers for $4500 a piece. Not sure how old they are at this point, but they both look very clean, have the WiFi controller, and the listing mentions it might have an ash auger system that isn’t installed.

So I guess I’m looking for opinions on these boilers, anything to watch out for or problem spots I should look close at? Buying two used boilers for less than the price of one new boiler doesn’t seem like a terrible idea. Also, does anyone have any opinions or experience burning something like municipal sludge? From the searches I’ve done, it seems to get brought up once in a while but not sure anyone ever follows through with it. If I don’t buy these boilers, I’d like to find a good used indoor pellet stove to experiment with the sludge pellets in the shop.
1-- Re-Post this in the boiler section, they will know lots more.
2-- You better research a lot more, before wasting money. From what I have seen, OWB's are not economical unless fuel is free.
There is a whole lot to burning sludge, hi temps, special procedures, special burners, because the pollution is incredibly high. And I don't think you will want to "do it right" and install "stack scrubbers" and other devices in required in your flue/chimney. And the regulations are usually very strict. I would not even want to handle the processed pellets, much less making the pellets.
I understand working in this environment is not for everyone, but most don’t understand how clean a waste water facility is (or should be). After being thermally dried class A bio solids are very clean and sterile. What is in the end product has everything to do with the municipality it came from. Our town has virtually zero industrial waste, so there is virtually no metals in the sludge. However, after a little research into permits, it appears you are correct an it cannot be burned without approval.

I did repost this over in the boiler section, as I am still curious about the boilers.
I have a maxim 255pe and have been pretty happy with it. You'll want to pay attention to the model of used maxims because as time has gone on they have made some pretty good updates to it. Most importantly in the realm of back burn protection. The 255pe has 3-4 backburn protection mechanisms in place that the earliest models didn't have. The 255pe also has an electric ignition where the earlier models relied on propane.

I can't speak too much to what to look at on a used boiler. But the maxim does have many more parts than a wood boiler and some of them are consumables like the burn basket, auger agitator, and the igniter. If I were buying used, I would look more at the burn box/water jacket condition. Make sure the electronics are in working order because the Maxim is useless without the electronics. I'd also check the augers.
On my first job, I looked into sludge drying and burning from a papermill. It did not make sense. Before you spend a lot of time, send out a set of samples to third party lab to have BTU content and analysis done. My guess is the BTU content will be quite low once assuming you exclude plastics.

My experience with municipal from long ago was that it was mix of primary solids and biological secondary. The primary might have burnable fiber but usually a lot grit along for the ride. The secondary sludge (usually pretty gooey) had minimal BTU content and then there were the "muffins" (lots of women's sanitary products has synthetic fibers in them) and other various plastics mixed in and that stream would need to be excluded as burning plastic in general can cause emissions and build up inside the boilers.