Most industries produce some amount of industrial wastewater. There has been continual pressure across a range of industries to develop processes that cut water consumption and wastewater. But many industries still use solutions that consume lots of water and produce high flows of wastewater. This article offers practical notes and useful guidelines on industrial wastewater treatment plants. The focus is on what to do — and what not to do — when dealing with industrial wastewater treatment units.
Unexpected Debris & Pollution
Operators often find large pieces of debris in industrial wastewater streams, with no idea where they came from. Processors should install proper screens and other provisions at the inlet of the plant. This is far better than having long arguments with operators and others to find the source (too often this is not possible).
Input from Other Experts
Input from the operation team, maintenance team, commissioning team and other major stakeholders is imperative. Often operators will ask for a specific filter/screen/strainer system in the inlet of a major unit, but the request is denied to save money. But later in the start-up stage, after many complaints from the operation team, management decides to provide what was originally requested by operators. The cost of such a provision in that late stage could be two to three times more than it would have been in the initial design stage (not to mention the unnecessary discussions, meetings, arguments and damaged relationships along the way).
The lesson learned is that comments from the operation team, maintenance team and commissioning team should be respected and resolved in a timely manner. For things like screens, suction strainers, filters and operational flexibility, operators usually have a more hands-on perspective than some design engineers. The same is true for constructability and commissioning issues, where the construction team and commissioning team usually have a better understanding of problems than the design team.
There have been long lists of operator requests at the end of a plant’s construction. Sometimes the list is so long that plant designers might as well go back to the drawing board. The best practice is to fully resolve concerns from all teams. This should be done considering overall cost during the plant’s life, not just initial costs. Another major risk is that a deviation, such as not providing a specific inlet filter, can affect subsequent units and downstream facilities. Management should consider all these factors and risks in the resolution of comments. It is always good practice to provide additional screens, strainers and filters on dirty industrial wastewater streams if budget, available space and operation allow.
Performance, Redundancy and Operational Flexibilities
Plant designers must ensure an industrial wastewater treatment plant can achieve expected performance. In some cases, plants have been demolished because they could not meet water quality standards. Management should take great care in selecting processes, methods and details of any unit, package and facility.
Proper redundancy should be considered for any system, machinery or key equipment. The plant should allow the maintenance of equipment — pumps, tanks, packages, etc. — while it is in continuous operation. The concept of sparing (or “duty plus one”) is important. To achieve high operational reliability, pumps and selected machinery/equipment (particularly maintenance-intensive items or those with relatively low reliability) should have at least one standby unit. This will mean that the plant can still meet its intended capacity with a unit out of service. This "n+1" approach usually meets redundancy requirements by many clients and operators ensuring redundant or standby equipment availability. In special cases, an “n+2” strategy might be needed.
For spare pumps and equipment, proper cycling and rotating should be considered during the operation. The performance requirements and operational procedures for the various systems should include cycling and rotating for all pumps, valves, membranes, basins and others. Operational flexibility should also be considered. Sizing of equipment, facilities, piping and even structures should meet the instantaneous peak flow as well as allow operation at lower average flow rates. For some plants, full flow may not be realized in the first few months or even years. Management should anticipate lower average flow rates during the initial months or years of operation. Treatment plants should be able to operate the minimum flow somewhere between 20% and 35% of the rated flow.
Future Expansion
To accommodate industrial growth, it is necessary to plan for future expansion of industrial wastewater treatment plants. Possible renovation/upgrading has always been a key requirement for such operations.
The usual expectation is to consider area, space and tie-ins in the design stage. Also, the plan should allow construction and connection of future expansion sections whereas existing units continue operation. Designers should provide proper tie-in points and tie-in valves in a way that existing units can be easily isolated and additional facilities can be installed without any interruption to existing operations.
Do Not Oversimplify
In many plants, produced industrial wastewater contains a diversity of impurities and pollutants. Furthermore, the emission limits for industrial effluent, whether to be delivered for other applications such as irrigation or released to environment, are constantly evolving. Therefore, industrial wastewater treatment is a complex and special task.
Wastewater can be contaminated by feedstock materials, by-products, product material in soluble or particulate form, washing and cleaning agents, solvents and many other constituents. These should all be considered in the design and operation of industrial wastewater treatment plants. They require sophisticated technologies, processes and facilities. Contractors and consultants who specialize only in municipal wastewater treatment units are not sufficient for these complicated and sophisticated plants.
Criticality of Neutralization
The neutralization (or pH adjustment) system is a critical step in primary treatment to make sure pH is in a proper range, as pH is a critical parameter of treated water. In addition, it is a vital step for proper operation and subsequent treatment stages, particularly major biotreatment systems. Neutralization is the process of adjusting the pH of wastewater through the addition of an acid or a base, depending on the target pH and process requirements. One of the critical items in neutralizing industrial wastewater is to determine the nature of the substances that cause acidity and alkalinity.
The neutralization tank is usually provided with a relatively short retention time, typically between 10 and 20 minutes. Neutralization tanks should be constructed with a corrosion-resistant material or lined to prevent corrosion.
The pH of the wastewater is measured by sensors installed at the neutralization tank and adjusted to be within the defined range (e.g., 6.5–8) by the pH controller, which controls the automatic operation of the dosing pumps (most often in “1+1” configuration). The pH control chemicals dosing units consist of chemical storage tanks, mixing tanks and mixers, and dosing pumps. For instance, caustic soda is usually received at high concentration (e.g., 48% or 50% concentration). Sulfuric acid is most often received at 98% concentration. Both are diluted to around 10% concentration for dosing purposes.
Neutralization frequently produces a precipitate that will require treatment as a solid residue. It may also be toxic. In some cases, gases may develop, requiring treatment for the gas stream.
Uncertainties & Risks
During the design of a treatment plant, there is considerable discussion and debate about the incoming wastewater characteristics to be assumed, primarily due to:
- Uncertainties related to the type of industries and manufacturing processes associated with the future use of the plant.
- Applicable codes and standards to be considered and their priority. Potential revision of codes should also be considered. Sometimes some local codes do not have any limit for certain specific parameters of treated wastewater.
- Considerable differences in parameters of actual industrial wastewater compared to specified values. Parameters include:
- Biological oxygen demand (BOD): the measure of dissolved oxygen required to break down biodegradable organics at certain temperature over a specific time period.
- Chemical oxygen demand (COD): the total measure of dissolved oxygen required for all chemicals that can be oxidized, so this also includes BOD.
- Total organic carbon (TOC): the amount of carbon present in organic compounds.
Conclusion
Technologies and processes for industrial wastewater treatment plants are complex. In industrial wastewater treatment, neutralization of excess alkalinity or acidity is usually required. It should be done in a knowledgeable and controlled fashion.
There have been many industrial wastewater treatment plants that could not achieve required performance and expensive changes were needed to make them operational within quality regulations for the discharge water. Some have even been decommissioned or demolished because the original designs and constructions could not be corrected. WT
About the Author: Amin Almasi is a lead mechanical engineer in Australia. He is a chartered professional engineer of Engineers Australia (MIEAust CPEng – Mechanical) and IMechE (CEng MIMechE) in addition to a M.Sc. and B.Sc. in mechanical engineering and RPEQ (Registered Professional Engineer in Queensland). He specializes in mechanical equipment and machineries including pumps, condition monitoring, reliability, as well as power generation, water treatment, and others. Almasi is an active member of Engineers Australia, IMechE, ASME, and SPE. He has authored more than 150 papers and articles dealing with rotating equipment, condition monitoring, power generation, water treatment, material handling and reliability.
