Design of Treatment Processes


Process design can be an intellectual adventure and an exciting flexing of engineering muscle.

When a waste manager 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 include 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.

Constraints

The process design 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

It’s not always easy to choose the best treatment 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.

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.

It works and when done correctly renders the waste free of infection. Efficiency of operation may refer to energy efficiency and operator manhours.

The big inefficiency results from too much stuff being put into the incinerator. Material handling processes and techniques should be considered when you are looking for ways to increase efficiency.

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

"Get a clear notion of what you desire to accomplish, then you will probably get itKeep 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."

Henry Maudslay’s Maxims

Design Q&A.

Green engineering for medical waste treatment.