September-October 1999

1. Understand the Composting Process

The US Composting Council's Compost Facility Operating Guide (CFOG) (1994) describes the seven steps of the composting process. Figure 1 is a diagrammatic representation of the seven steps and how they relate to one another.

During the composting process six variables must be managed to optimize composting conditions. CFOG refers to them as key process variables for pile management:

1. Pile structure/porosity

2. Initial pile nutrient balance

3. Pile oxygen percent (and pH)

4. Pile moisture percent

5. Pile temperature

6. Retention time

Figure 2 from Appendix B of the CFOG identifies with check marks where management of the key process variables is required in each of the seven steps of composting.

A comprehensive understanding of all the composting steps and of the key process variables to manage is essential for choosing the right equipment to use during each step.

There are many opportunities available to assist you in understanding the composting process. You can study textbooks and references, attend university courses or programs, or attend training workshops.

Training workshops can offer the advantage of transferring the fundamental principles of the seven steps of the composting process and of key process variable management in a relatively short amount of time. Workshops typically range from one-day short courses to five days with hands-on training in pile building and management.

2. Understand the Functions That Equipment Is Required to Perform in the Process

For the purpose of this article, let's look at an example in which you have to select pile-turning equipment. We'll also make a few assumptions. First, let's say that you attended a week-long workshop and you now understand the seven steps of composting and the importance of managing the key process variables. By attending the workshop you will also understand what the pile-turning and mixing objectives are that need to be met in the "composting step" of the composting process, as described in the box below.

Next you need to determine from the above list of objectives what functions the pile-turning equipment must be capable of performing. For instance, since one of the pile-turning and mixing objectives is to restore and maintain porosity, then the turning equipment must function to break-up clumps, break-up air channels, and redistribute bulking material while avoiding size reduction of particles in the pile.

All seven steps of composting should be considered in determining the functions for the pile-turning equipment, and a list of functions should be compiled.

3. Describe Equipment

Function Requirements

The list compiled in Step 2 must be compared to the control limits established for the key process variables listed in the CFOG. This will define specification limits for each required equipment function.

For example, we know that our pile-turning equipment must restore and maintain porosity. However, the specification to potential equipment vendors needs to be more specific. Otherwise, when asked if their equipment will restore and maintain porosity, the answer will generally be a subjective "yes." However, after researching the control limits for key process variable management, you'll expand your specification; in this case to say that the pile-turning equipment must restore and maintain the porosity within the range of 45-60% (during the composting step). The specification must also list a measurement standard such as those found in the Composting Council's new Test Methods for the Examination of Composting and Compost. Your specification should request that a vendor's proposal include porosity measurement data from a compost process similar to yours from a reputable laboratory referencing the correct test method employed. Now with a complete specification (for porosity) as described, a vendor's response can be objectively evaluated (based on the data supplied).

Once the technical requirements for the pile-turning equipment are established, the specification needs to be expanded with other required information, such as:

• Price
• Delivery
• Operator requirements
• Terrain and environmental conditions
  • weather
  • hard surface
  • support rails on pile sidewalls
• Ruggedness and durability for
  • pile height and width
  • pile bulk density and moisture percent
• Pile span and system
• Energy input to pile
• Maintenance and parts
• Repair complexity
• Turning capacity rate
  • travel advance rate (speed)
  • cubic yards, tons per hour
  • combined capacity (advance and turn) per horsepower at specified pile bulk density, configuration
• Associated equipment:
  • tow tractor, loader
  • water wagon
  • carriage
• Vehicle turning radius
• Associated equipment for makeup water storage, transport, application
• Over-the-road capability

4. Solicit Equipment Proposals

After you have generated a detailed specification for the pile-turning equipment, you need to solicit proposals from equipment vendors. A good place to start is an annual equipment directory, which will provide a list of potential pile turning-equipment vendors.

Before mailing your Request for Proposal (RFP), you might want to have an experienced consultant review it and your list of prospective vendors. He might quickly identify relevant information overlooked on the RFP, missing equipment vendors you should be looking at, or other overlooked areas.

Be prepared to answer questions from equipment vendors who are unsure of exactly what information you are looking for.

5. Evaluate the Proposals

Once you've received the vendor responses, you need to compare them to your specification. You might need to contact vendors to clarify their response. In the process of your review, you'll probably start to develop a list of interesting proposals. But the single most important factor that should influence your selection is how the specific equipment will affect the composting process. That is, will the equipment do what you want it to? There are a number of methods to aid you in your evaluation.

First, make sure the equipment vendors you're dealing with understand the composting process. If they don't, how can they understand how their equipment will affect it? In other words, if the vendor doesn't know what porosity is throughout the composting process, then how can he predict how his equipment will impact it during different stages? Most likely he can't.

Why is this important? Let's look at a case study. An Owner is looking at three vendors of compost pile-turning equipment. They all claim that their equipment will turn the required amount of material in the allotted time. They also claim that the equipment will maintain porosity, moisture, and temperature, as required by your process.

After reading this, the Owner makes equipment inspections at one or more operating facilities. He observes that Vendor A's machine maintains porosity and moisture, but when it turns the material on the designated cycle, the temperature drops so far and recovers so slowly that pathogen reduction criteria for time and temperature would not be met if installed in his particular application. The Owner observes that Vendor B's machine maintains pile moisture and temperature but pulverizes the material to the point where pile porosity isn't maintained. The Owner observes that Vendor C's equipment maintains pile porosity, moisture, and temperature as well as processes the required amount of material in the allotted time. So in this case, a clear technical choice is easy to make. But remember that it's not always that easy.

Second, as indicated by the above example, make sure you see the proposed equipment. Try to see both a new/unused machine and one being operated at a full-scale composting operation.

Viewing new/unused equipment allows for a more thorough inspection of the machine's features without interference from dirt buildup and allows for a point of reference to gauge how well the equipment holds up to use when compared to equipment that has been in operation.

Viewing equipment being used at a full-scale composting facility gives you an idea of the practical side of the equipment's operation. How well does the equipment hold up as displayed by many factors, such as the level of corrosion exhibited over time (was the painting system adequate?) or the level of physical damage exhibited over time (was the equipment built sturdily enough?).

Third, when visiting a facility that uses the same equipment you're considering, talk with the facility operators and get their feedback. Try to ascertain their knowledge of the composting process to ensure that your frame of reference, the science of composting, is the same. Ask them if the equipment performs in the process as expected. In the case of your compost turner, is it able to maintain porosity, moisture, and temperature? Ask them how well the equipment works for them, what they like and dislike about it, and what type of operating or maintenance problems they've had with it. Ask them about the vendor support they've needed and received, as well as any warranty issues they have faced. Finally, ask them if they would buy another one should the need arise or if they would buy something else and why.

6. Negotiate a Purchase Agreement

At this point, your inquiry specification, the vendor's proposal, and the verbal discussions with the vendor during the evaluation need to be merged into a single document that defines exactly what you are agreeing to purchase. The buying agreement must also include price and delivery and all details of the transaction, including such matters as guarantees, payment schedule, retainer, shipping details, drawing exchanges, shop inspections, operating and maintenance manuals, installation and commissioning, spare parts and service, training, and so on.

The equipment selection process is one of the key steps in developing a successful composting operation. By approaching the task of selecting equipment in the manner outlined in this article, you'll have a sound basis for making an informed choice. An informed choice will allow you to predict and not guess your future success.