Design of Treatment Processes


When a waste engineer designs a treatment process, he or she is establishing the sequence of chemical and physical operations needed to produce the desired effect. The design also includes instructions for operating conditions of the equipment and quantities and flowrates or throughput rates. For instance, temperature; electrical loads, target pH, residence time of materials in vessels are all written down as instructions to guide the construction and operation of the treatment process.

Development

Engineering guru Alexander H. Slocum says "design is a series of steps blended together." He means there are many factors and mental processes required and design is almost always iterative.

The first step is to develop alternatives. Sometimes these alternatives taken together are called a process topography. You can develop a flowsheet for each process. The flowsheet identifies major equipment and material flows. There are two types of flowsheets: block flow diagrams and process flow diagrams.

Documentation

Flowsheets/flow diagrams are fundamental engineering documents used by everyone on a facility. The terminology used by different companies and industries varies somewhat, but the general hierarchy is from block flow diagrams (BFDs) to process flow diagrams (PFDs) to piping and instrumentation diagrams (P&IDs).

Block Flow Diagrams

On these diagrams each major operating step is represented by a block. Lines connect the blocks to indicate the flow of material between blocks. These are great for high level communication and letting people (including regulators) know what you are intending to do in your process.

Process Flow Diagrams

Mass and Energy Balances
Or material and energy balances. The word "balance" is widespread in process engineering and it is similar to the balance seen in accounting. It means keeping track of stuff. In - out = accumulation. In the long run, accumulation is zero, so what goes in comes out. The process might change the nature of the stuff. Chemical, physical processes can change things, and indeed this is “why” you are running the process. But, you can do balances on:
  • Energy in = energy out
  • Total mass in = out
  • Each element. For instance, carbon in = carbon out

A PFD contains process information for all significant streams. This information typically includes flow rates, chemical compositions, phases, temperatures, and pressures. The PFD shows major control systems, but not instrument detail. The profession of chemical engineering (or process engineering) has norms. By custom, the following information shows up on a PFD:

  • Major equipment and how they are connected
  • Control valves and process-critical block valves
  • Bypass and circulation lines
  • Process piping above a certain size
  • Connections to and from other PFDs (if any)

The flow of major streams goes from left to right. The top of the PFD is “up” in real life: light streams (e.g. vapors) exit tanks from the top, and heavier streams from the bottom. Distances are not shown or implied. The PFD includes a pictorial representation of major equipment but the relative size of the equipment pieces to each other is not necessarily to scale.

Constraints

The process design is often constrained by government permits or regulations. Other major factors include demand/desired throughput capacity, availability of power, steam, data lines availability of water/drains/sewage, and whether a building exists that can house your new process.

Information

Accurate data can help you improve and optimize your treatment process, and it can be invaluable in helping you come up with new processes or additions to your existing process.

That’s one reason we encourage waste managers to keep good records on how much waste you make (by category), how fast you make it, and how fast you can get rid of it. Think like an accountant.

Selection of technology

Engineers consider factors such as expected:

  • Effectiveness

    Does the process render the waste acceptable for disposal in a landfill? These include sanitary landfills and low-level radioactive waste landfills.

    How much the volume of waste will shrink. - Disposal costs scale with volume and sometimes mass.

    Production of secondary waste. Most processes do produce secondary waste, so it is not a deal-killer itself. The question is how much secondary waste is made and how hazardous or difficult it is to deal with.

  • Costs

    Capital cost - including cost of installation, additional building costs (if any), instrumentation (for controlling and monitoring the process), and a contingency cost.

    Operating cost - including realistic labor costs, cost for cooling water, electrical energy, natural gas (if used), any chemical additives. A lot of judgement calls are needed when estimating operating costs (e.g. whether employee cost will be assigned fully to the waste process or only part of it.)

    Overall cost analysis requires taking time into account, and will require an estimate of equipment lifes, future replacement and maintenance costs, and the cost of capital.

  • Capacity

    Will your process be able to take in waste at a sufficient rate? Is there a cushion in case the needed rate is higher than you expect?

    Is the capacity going to be appropriate in the long run? For instance, if your facility expands or you need to take additional waste on a regular basis, your waste treatment rate could increase.

    Does the facility have adequate utilities (e.g. water, electric power).

  • Reliability

    What are the costs and threats of equipment failure? Do you need a back-up or contingency process?

    When a unit goes out of operation, can the rest of the process function? How difficult will repair be, in terms of manpower and time?

  • Safety

    How dangerous will the treatment equipment and process be, and can you mitigate the risk? Heated machinery poses risks to employees. So do certain chemicals.

    What worker safety and ongoing equipment education will be needed?

  • Other questions

    Is this a well-honed technology? Has it been around for decades and used by dozens or hundreds of waste treatment outfits? Or is it new? Pro-tip - if you are going to employ a new process unit, do only one new unit at a time. Make the rest of the process old, established technologies. Consider asking equipment vendors for references.

    Is the process expected to be complex or easy to operate?

    Will the public object to the process?

    You can also ask for advice from paid consultants (which can be pricey) or, people you might know at other treatment sites or through professional organizations.

Regulatory considerations

What treatment processes will be easier to get approved? Which ones will require less communication with regulators in the future?

Other Criteria

Safety of workers

Generation of secondary waste and environmental impact

Volume reduction and its impact on disposal costs

Throughput capacity

Available building/water/drain/electric power

Upfront costs vs ongoing costs

Complexity and expected difficulty in operating

Objections and opinions of public

Regulatory requirements and how easy it will be to get a permit.

Engineers have developed heuristics to help with selection and sequencing of unit operations. Sometimes the designers will have existing equipment to work with and will be tasked with figuring out what to do with that equipment and what new additional treatment elements to add. Designers usually produce a process flowsheet with schematic pictures of process equipment and lines showing transfer routes (pipes or otherwise). A mass and energy balance sheet will probably be attached. These are valuable for determining the needed size of the equipment and for communicating to regulators and stakeholders how the treatment process will operate.

Sometimes the design process will bring to light information that is missing or insufficient. This is valuable because it prompts the waste management engineer to find that information.

Process Equipment

Equipment is often subject to codes and standards. These rules are set by government agencies, insurance companies, and professional organizations. Even when there is no law about your equipment, you should follow industry standards to avoid liability risk in case things go wrong.

process valve Equipment manufacturers sell set “off-the-shelf” equipment with set capacities and sizes. Often these are listed on the manufacturer’s website and prices are fixed. Pumps, filters, and agitators for mixing are in that category. Custom-made, or customized standard equipment, are not fixed price and you need to get a quote from the seller. What do you have to tell the seller? He or she will want as much information as possible, including intended operating conditions and capacity as well as utilities available in the facility (steam, cooling water, electrical outlets, etc.)

Efficiency

Is medical waste treatment inefficient?

Yes, probably, but mostly because too much waste is classified as infectious. This is caused largely because the people who manage medical waste err on the side of safety The process of treatment – usually heat treatment/incineration is not necessarily inefficient.

Developing a medical waste management plan.

Process Validation

Once the treatment process is up and operating, you should test it to make sure it operates as intended before you start running it on a regular basis. Validating the process can give you peace of mind as well as being a great way to encourage any regulators to give you needed permits. The validation establishes that the process achieves some benchmark:

  • Destruction of microorganisms (to some level)
  • Destruction of target contaminants or compounds
  • Operating temperatures and pressures required to get to those destruction levels.

Philosophy

"Design is conceiving and giving form to artifacts that solve problems" - Karl Ulrich

How is waste treatment process design different from other chemical process design?

Conventional

  • Purity of final product important
  • Market price of final product(s) and production costs set limits on economic viability of process.
  • Depending on nature of process and materials involved, safety may be very important or less important.

Waste

  • Purity of final product NOT important
  • Economics important, but there is no sales revenue to balance against costs. Costs of running the process and cost of final product disposal are paramount.
  • Safety is generally more of a concern.

How does the design of a medical waste treatment process differ from the design of any other waste treatment process?

Most of the time it doesn’t. The principles are largely the same: contain, don’t increase volume or mass of waste unnecessarily, protect workers, the public, and the environment.. Neutralize harmfulness of waste constituents. The only exception is when there are disease vectors around, and even then other waste disposal processes are subject to attack by vectors like rats.

The designer must be aware of the risks and hazards posed by the waste and anything generated by the process. And these risks and hazards can vary considerably.

Henry Maudslay’s Maxims:
  • Get a clear notion of what you desire to accomplish, then you will probably get it.
  • Keep a sharp look-out upon your materials: Get rid of every pound of material you can do without.
  • Put yourself to the question, "What business has it there?"
  • Avoid complexities and make everything as simple as possible
  • Remember the get-ability of parts

Design Q&A.

Green engineering for medical waste treatment.