For most industrial facilities, the engineers’ core competencies relate to the equipment used in the manufacture of goods and materials. Installing, or upgrading a wastewater treatment system is an infrequent requirement, and engineers find themselves learning as they go along. This can lead to ineffective communication between plant engineers, process consultants, equipment manufacturers, and regulatory authorities.
The first step in planning a wastewater treatment system is to determine what treatment processes and equipment are suitable for you. The second step is to compare treatment processes against each other, taking into consideration the context and operational realities of our individual plant.
This Part 1 article will review the planning process to determine a suitable treatment process while a follow-up article, Part 2, will review site specific factors that will guide your decisions on individual treatment options.
There is a comprehensive way to organize our thoughts when seeking a new wastewater treatment process.
When we evaluate a new wastewater treatment need, there are a series of questions we can ask. At a high level they are:
- How much water do we have to treat?
- Where is it coming from?
- What is in the water?
- Where will it go after treatment?
- What are the treatment objectives? (What does the water need to be like after treatment?)
How much water do we have to treat?
A simple question at face value, but after we establish a figure like gallons per minute (GPM) or gallons per day (GPD). We need to dig a little deeper into the operational schedule of the plant and the duty cycle of the equipment producing wastewater.
Operational Schedule
The operation schedule of the plant will allow us to determine during which hours staff are available to operate equipment (unless equipment can run unattended). The number of days per year which the plant will operate will be important to understand the total yearly production of waste treatment byproducts like metals sludge, and the yearly consumption of reagents, power, and fresh water the system will consume. These figures will be important later in developing the operational cost of a given system.
Duty Cycle
The duty cycle of the equipment producing the wastewater tells us how much water is produced in what period of time. This factor is a description of the daily variations in flow that may not be understood from the gallons per day or peak gallons per minute.
For example, a waste stream may have a flow rate 10 GPM, but this process only runs for 6 hours per day and the plant operates 24/7. In this case it may be advisable to install an equalization tank with a capacity to hold an entire day’s water production, and install a treatment system ¼ the size, treating the water 24 hours per day, even when the water is not being produced.
Another important consideration of duty cycle is the tolerance of the treatment technology for stopping and starting. Technologies like ElectraMet and sand filtration are very tolerant of stopping and starting operation multiple times per day if necessary. They are also tolerant of sitting for long periods in standby. Other technologies are meant to run most if not all the time. Frequent on-off cycles and periods of sitting have a negative effect on membrane life and water efficiency in the context of cross flow membrane treatment technologies like reverse osmosis and nano-filtration. Biological treatment systems employ reaction tanks populated with living bacteria that need a steady state flow all day and all night, all year round to remain operational.
As a starting point of answering, “How much water do we have to treat?”, we recommend collecting the following information:
- Flow rate in gallons per minute (GPM) of the water being produced.
- Total gallons per day (GPD)
- Operational schedule of the plant (Days of the week?, Hours?, Days per year?)
- Describe daily or weekly variations in flow that may not be captured by the peak GPM or gallons per day.
Where is the water coming from?
This questions seeks to understand two aspects of the wastewater:
- Which specific industrial processes are producing the wastewater?
- Is the waste stream coming from one process or a combination of several smaller streams?
In the early stages of evaluating a wastewater treatment process it is not uncommon to be missing important water characterization information describing the contaminants present and their concentrations. If the consulting engineer has a description of the upstream manufacturing processes, they may be able to make assumptions based on previous experiences.
For example, if the wastewater is a washdown of dairy equipment we might assume that separation of fats and oils will be required prior to further treatment. We might also guess that biological digestion of excessive BOD may be required, or desirable depending on the limitations and surcharges of the receiving sewer authority. In the semiconductor industry we may be able to presume that certain etch and electroplating processes will have a higher concentration of copper and that rinses may have a lower concentration of copper. Chemical mechanical planarization (CMP) processes upstream alert us to the likely presence of colloidal silica.
Understanding the source of wastewater production may help theorize treatment paths at early stages where a complete understanding of the stream is not available.
The second part of understanding “Where is the water coming from?,” is identifying individual streams contributing to the wastewater, and understanding their flow and characterization. (To simplify, what streams?, how much water?, and what’s in it?)
Many industrial plants collect wastewater generated from multiple processes in a single location and treat the entire stream before discharge. This is not the most efficient method in all cases.
Take for example a facility with 10,000 GPD wastewater from demineralizer regenerant waste, 40,000 GPD cooling tower blowdown, and 1440 GPD rinse water from a copper electroplating process. If the only stream exceeding the discharge limit for copper is the 1440 GPD electroplating process, and the demineralizer regenerant and cooling tower blowdown are dischargeable without treatment (commonly are), then it would make more sense to point source treat the 1440 GPD electroplating process with a much smaller system.
We often see examples where a very minute stream of concentrated waste is the sole cause of exceeding discharge limitations. If this volume of water can be separated, and is of relatively little volume, it may make sense to have it shipped off site for treatment.
To fully consider this aspect of wastewater planning we recommend creating a list of wastewater streams, their volumetric flow, and a brief description of the process which creates them.
What is in the water?
This question might seem obvious, but understanding the water characterization is often difficult. It is not uncommon to make final changes to system designs in the late stages of the project to better match a new understanding of the water and its contaminants. Several common errors occur when trying to generate a water characterization.
Problem 1: Only Testing for the Regulated Metals/Chemicals
This error occurs when a plant operator is told by the sewer that they must treat for a specific set of metals, or told that they are exceeding the discharge limit on those metals. The client then takes a sample of the water and lab tests for those metals they have been told to treat.
This effort falls short, because not only must we understand the concentration of the contaminant we wish to remove, we must also understand other contaminants which may affect the treatment process.
For example, a customer is over their limit for total suspended solids, but they don’t test for TOC or BOD (a measure of organic mass in the water). A sand filter may be a fine solution for the suspended solids but may form a bacterial science experiment in the vessel with a high presence of organic matter. In this case a clarifier may be more appropriate.
In an example of a metals wastewater, there may be a chelating agent present, which will interfere with the precipitation of metal hydroxides, and place a lower limit on the concentration of metals that can be achieved.
Problem 2: Analyzing A Sample That Is Not Representative
This problem can occur in several ways.
First, you can take a sample during a time when an anomaly upstream has caused the wastewater sampled to be atypical.
Second, you can analyze a water sample that is typical, but does not alert you to the water conditions during anomalies upstream that could cause your treatment process to fail.
If you suspect a high degree of variability in your waste stream it is best to take and analyze multiple samples over both time, and different plant operation conditions. Don’t average these results. Averaging multiple samples provides the process engineer with a characterization of water that never really existed and obscures peak conditions for certain contaminants. It is best to present the wastewater treatment engineer with all the sample reports (well labeled) or at the very least present the analytes as a range.
The last, and extremely common occurrence, when characterizing water is taking a sample from the wrong location. Plant operators are most familiar with taking samples at their sewer outfalls after water has undergone treatment so they may report results to the authorities.
But what if we are evaluating a replacement treatment system? In this case we would need a sample of water collected before the existing treatment system. In some cases we may want to intercede with a new piece of equipment in between two existing pieces of equipment, maybe after the clarifier but before the sand filters, and so this is where we need a sample.
The best way to think about sampling is to take the sample of the water at the point where it would enter the new water treatment system.
How do we determine what is in the water?
To understand what is in the water we can look at the processes upstream that are producing the water. Make a list of chemicals and materials that are used in the process and review chemical data sheets for ingredients. This will give you a good starting point of knowing what might be in the wastewater, but not the concentrations or if it will be a problem.
The second step should be sending the sample out for laboratory testing.
As a starting point for a laboratory analysis, we recommend you test for:
- TSS (total suspended solids)
- TDS (total dissolved solids)
- TOC (total organic carbon)
- BOD/COD (if organic treatment is likely)
- Hardness
- Alkalinity
- Silica
- Bromide
- Chloride
- Fluoride
- Free Chlorine
- Total Chlorine
- Nitrate
- Nitrite
- Sulfate
- All individual metals (cations)
This level of analysis can usually be obtained for under $1,000 per test. It gives a strong starting point for identifying both contaminants that may be regulated in your discharge and understanding what contaminants may interfere with some treatment methods.
**An important addition to this list would be to add testing for any contaminants that you reasonably believe may be in the water and that you will be regulated for.
For guidance on what to test for it may be helpful to consult with a systems provider like ElectraMet before sending out for testing.
Most cities will have a local laboratory capable of this type of testing. National suppliers of these services include National Testing Laboratories, and Pace Analytical.
Where will my water go after treatment?
Wastewater will go to one of four places after treatment. Knowing its destination will let you determine how it must be treated, and what your limitations on contaminants will be. The final destination for industrial wastewater will be:
- Sewer (referred to as Publicly Operated Treatment Works or POTW by industry professionals)
- Environment (discharge to the ocean, lake, river, stream, intermittent ditch, or percolation pond)
- Water Reuse (reuse treated water in the plant)
- Evaporator/Crystallizer (Zero Liquid Discharge ZLD)
Discharge to Sewer
The sewer is often the most advantageous discharge location if available. While the sewer authority will charge for the service, it allows the water to be received and treated in bulk, and also eliminates the need for the plant to be located adjacent to a body of water.
Sewer discharge requirements vary from district to district. There may be different discharge limitations within the same sewer authority depending on the specific treatment plant you discharge to.
When applying for an industrial sewage discharge permit it is important to collect the information described in the beginning of this article, and it may be beneficial to engage a local environmental consultant with specific experience working for your local sewer authority.
Discharge to Environment
If a plant is located next to a body of water, it may be more economical to discharge directly to the environment. Discharge to the environment requires a permit under the EPA’s National Pollutant Discharge Elimination System (NPDES). Additional state and local level permits may apply depending on your location.
Engaging an environmental consulting engineer to assist in applying for this type of discharge is strongly recommended.
Water Reuse/Recycling
If sufficient treatment can be achieved water may be recyclable back into the plant. This is particularly valuable for areas with high costs of water supply and disposal, or areas with limited water resources.
Evaporator/Crystallizer
An evaporator/crystallizer applies heat to water evaporating and recovering water condensate and concentrating up contaminants in a recirculating stream. As the water is removed from the stream through evaporation, dissolved contaminants reach their solubility limit and begin to form crystals. The solid crystals are centrifuged out as solids and sent to solid waste. This option is by far the most expensive from both a capital and operational standpoint. It is generally only considered when discharge or reuse are not options, or there is an extreme need for water recovery.
Treatment Objectives:
Treatment objectives describe the condition of the water after treatment, or what can be left in the water after treatment. They are generally expressed in terms of specific analytes being less than a specific concentration in parts per million (ppm), or milligrams per liter (mg/L).
In addition to maximum concentrations for specific analytes, some discharge limitations will also have figures for average concentrations and daily or weekly maximum. There may also be a maximum total mass allowed for certain substances, meaning the volume of the discharge will need to be considered along with the concentration.
Treatment Objectives for Crystallizers/ZLD
The least common destination for treated wastewater will be an evaporator/crystallizer. This technology uses heat to evaporate and recover water until contaminants reach a concentration higher than their solubility limit and begin to crystallize into solids. The water is recirculated through a centrifuge separating the solids as dry sludge or powder. This technology is energy intensive and the equipment very expensive.
Planners should consult with the equipment manufacturers to receive specifications for the water that is allowed to enter the system. Pretreatment of this water is usually necessary. In addition to meeting the requirements for the crystallizer equipment, opportunities to reduce the volume of material entering it, either through segregating dischargeable streams, pretreatment, or pre-concentration, should be evaluated. This will reduce the size and cost of the equipment and decrease energy consumption.
Treatment Objectives for Sewer (POTW) Discharge
When discharging to a sewer we look at the sewer discharge permit, or for a new facility a provisional discharge guideline. The local sewer authority may publish standard guidelines, or an environmental consultant with local experience can be consulted.
Treatment Objectives for Discharge to the Environment
Contaminant limitations for discharge to the environment are generally the most stringent. When discharging to the ocean, lakes, or streams industrial waste is mixed with water communities rely upon for recreation, drinking, and fisheries. Furthermore, it does not have the benefit of municipal sewer treatment, and dilution from other less toxic waters from domestic sources.
The treatment objectives for water discharged to the environment will be determined by the facility’s NPEDS permit, and any local permits or regulations. These can be obtained by referencing an existing permit, provisional guidelines from the EPA, or by consulting an environmental engineer with experience in this subject.
Treatment Objectives for Water Reuse
Water reuse is an increasingly common and desirable destination for treated wastewater. For water reuse there are two places we should look to understand treatment objectives. First we should identify the equipment that will use the water, and consult the manufacturer of this equipment for the water quality requirements. Some likely destinations for water reuse include cooling towers, rinse waters, scrubber spray, and application for dust control. The treatment objectives for water reuse will be the water quality requirements for these applications.
Some facilities will have water purification equipment treating incoming municipal water or surface water (lakes, rivers, etc.) to meet process conditions. Sending treated wastewater to the front of a plant for purification is an excellent option. For this use it is important the wastewater be treated to at least the quality of the incoming fresh water. This will allow the plants fresh water purification systems to treat the water in the same manner as if they had received it from the municipality or surface water. In these applications it is important to prepare a water/mass balance to ensure that contaminants will not be concentrating up in the closed loop over time.
Getting Your Project Started:
In preparing this article we have erred on the side of providing a reasonably comprehensive review of information to consider when developing an industrial wastewater treatment process. In the reality of industrial projects we have never encountered an opportunity where all of this information was presented to us in complete and final form.
We hope that this article will assist you in gathering or at least understanding what information will need to be gathered during the planning process. We would, though, encourage you to reach out to system providers like ElectraMet early in your planning process, regardless of the completion of information you have available. Our experienced staff of sales executives, and sales engineers welcome the opportunity to guide you to an optimal outcome for your industrial wastewater project.