Poop Digester Version 2: Introduction

The first semester digester project, presented here ended in December, 2008. The research group needed a fully-featured manure digester to get their work done, and I needed to make headway in my research as well.
This digester combines knowledge from the previous semester in manure digester design, coupled with site visits to a couple of state-of-the-art digester facilities in the North Country. The result, after about eight weeks of work, is the Clarkson Anaerobic Manure Digester. This machine is a fully equipped digester. The only original parts left from the previous semester that weren't removed, cleaned, re-attached and re-designed was the tank itself. The trailer even got a new set of wheels. Nearly all of the construction work and most of the engineering was done by myself, Shaun Jones, and Dan Valyu. There's a photo of the three of us later in the document. Thanks to the Biomass Group and the great shop staff at Clarkson who were helpful and available, and taught me how to weld.

Layout diagram The component layout of the new digester is shown at right, looking top down in schematic form. At time of writing, in June, 2008, the system is up and running, taking data for the summer research at Clarkson. For further information, feel free to download the digester's instruction manual The manure handling system is all 3 inch schedule 80 PVC or galvanized steel. All joints that may need to be removed are bolt flanges, The pump is installed with bypass throttling and a pump reverser, to enable us to mix the tank backwards if we need to remove sediment. Thanks to Dan for that suggestion. The manure system is equipped with lots of extra valves, to enable us plumb the recirculating pump to various external systems. The system is also equipped with an ultrasonic flow meter on the manure plumbing. The water heater system has been redesigned. Originally a tank-based system with bang-bang control, it was replaced with a dual-control loop flow through design. One loop controls the tank temperature, and the other controls water temperature. The water pump was also replaced with a pump more suited to the job of circulating heating fluid. This saved us energy, and enabled a reliable, repeatable flow rate (16 GPM). The tank also has a support frame around it. The frame supports the electronics enclosures, water heating system, gas system, and conduit and wire. The frame makes it much easier to work on and install things, compared to the previous way of attaching things to the lid.

Side view A view of the digester in the shop at Clarkson, showing the system from the side. All key parts are labeled. At the right is the circuit breaker panel and the manure ultrasonic flow meter. The bolt flange immediately to the right is the effluent output. The top of that pipe is transparent to the weir itself, which has an open top to prevent siphoning the tank when the system is drains. The tank pressure was set to be around 9 inches of water when full.
Continue going to the left, the gas system meter and pipe is at the top, all continuously heated to keep water vapor from condensing out of the gas. The HMI LCD panel is also visible, and a side view of the manure system. Each of those big blue valves is around 60 lbs. There is a section of pipe wrapped in gold material. That is the manure inlet heater, designed to compensate the tank temperature for the inclusion of a slug of cool manure when being fed. The tank holds around 515 gallons total, of which 45 is in all the pipes and plumbing.

Back view The back view of the digester. Left is the AC cabinet, containing contactors, overloads, and AC distribution. Right hand is the PLC cabinet, with contains the SixNet hardware and DC power supply. Up the center is the access ladder, which was absolutely wonderful to have when welding inside the tank. The main drain is visible at the bottom of the frame, with a 3 inch pipe coming out of it. At the far right of the frame is some of the copper of the heating system.
As with the previous digester, all the electrical equipment is mounted in NEMA 4X or 3R enclosures, where applicable. All cables are pulled in Sealtite or other weatherproof fittings, and AC and DC equipment are separated in different enclosures, to preserve the signal integrity of the DAQ channels, and make data downloading and PLC maintenance safer.

AC cabinet Inside the left hand enclosure, showing the circuit breakers, contactors, and associated hardware. At left is the 24V DC distribution, next to the 240V AC. I would rather have not done it this way, but due to time and money I was re-using a lot of the old conduit and enclosures from the previous version of the digester, and it just happened that the 24 V DC all wanted to come together here. At the lower right is the transformer for the heater pump. Due to time constraints, a 240 V heater pump could not be located in time, so a 120V pump with a step down transformer was used instead. We are till operating safely within the pump's rating. Only four of the contactors are currently used, from left to right, the big manure pump, the heater element, the heater pump, and the manure heater. The rest of the contactors are for the future systems to be integrated with the digester's controller.

PLC cabinet This is a photo of the PLC enclosure. This cabinet contains the 24V power supply for all the low voltage hardware, much of the analog input hardware, and all the digital IO and the PLC itself. There is another set of A/D modules mounted next to the HMI, to save thermocouple wire pulling headaches. SixNet was very nice about giving us a good deal on this hardware, and it has been a pleasure to work with.

AC control panel This is a photo of the control panel itself. These controls are located on the front of the AC panel, even though most of the wires run back to the DC panel. Once again, this is due to the re-use of hardware from the previous digester. The top row of knobs are wired for auto/off/on type operation. The bottom row of pushbuttons are for semi-automatic operation. This allows us to have asynchronous feeding times at user discretion, while still running the pumps and other systems in a controlled manner. One of tricks of this was to keep the current down for use on a 30 A 240V farm electrical service. These pushbuttons enable that to happen.

LCD Menu screen formats This is a view of the screens available on the HMI. I wanted to make it as easy as possible to configure digester options and see the real time data. The DAQ system takes a sample of all parameters every 10 minutes, but I wanted a way for the operators to tell what was happening in real time. The screens are divided to enable monitoring of the manure, water, and gas systems, and set the mix and heat parameters. You can also view the tank temperatures in real time. A high level screen showing key digester health parameters is available in the overview button.