How to choose a water treatment system
CLICK IMAGE TO ENLARGE (+)
Water is used in almost every industrial manufacturing process, including plating and other finishing processes. A necessary evil resulting from this water use is wastewater, which every plant manager has to deal with. Wastewater is produced either from the manufacturing process itself or from the cleanup of manufacturing equipment and parts.
When the Clean Water Act was adopted in 1973, a new industry was created out of a need to deal with this wastewater. Several wastewater treatment technologies were developed. The simplest used a gravity settler and interceptor installed just ahead of the floor drain.
As process chemistries became more complicated, the wastewaters generated from these streams also became more complex and more difficult to treat. At the same time, regulations for discharge to sewer and surface water were becoming more stringent. The need for more sophisticated wastewater treatment systems forced the invention and development of more wastewater chemicals and equipment. Today, companies looking for wastewater treatment technology can choose between mechanical or chemical treatment systems or a combination of both.
Mechanical wastewater treatment systems consist of separation, filtration or evaporation equipment.
Separation equipment can encompass air flotation systems, gravity settlers, centrifuges and coalescers. Air flotation, gravity settling and centrifuge technologies work very well for separation of suspended solids of specific gravity higher than water. Coalescers work on a partitioning effect: Small particles are attracted to and collected on a coalescing media until they create a particle that is large enough to float (in the case of oil) or settle.
A variety of types of filtration systems are available. These include gravity bed filters with disposable media, pressure filters with disposable filter cartridges or back-washable sand, filter presses with reusable cloth filters, rotary drum vacuum filters with a diatomaceous earth pre-coat and membrane filters.
Evaporation systems consist of insulated tanks that are constantly kept at elevated temperature. This allows the water phase of the wastewater to evaporate, leaving the solid phase as a concentrated waste.
Chemical wastewater treatment systems can be split into three broad categories: metal salts, polyelectrolytes and clay-based flocculants. In most cases, systems are set up with some combination of the three.
Most chemical systems are set up to allow pH adjustment by using an acid to lower pH or caustic soda to raise it. pH adjustment has two basic purposes: to break oil emulsions at a low pH or precipitate soluble metals at a high pH.
Once this pretreatment step is complete, one of three categories of flocculants—metal salts, polyelectrolytes or clay-based materials—is added to collect the small particles that have been generated and ultimately get these larger particles to float or settle. Metal salts such as aluminum sulfate and ferric chloride are commonly used as cationic flocculating agents to neutralize negatively charged ions and form them into a large, filterable state.
Polyelectrolytes are long-chain polymer systems produced with different charge densities of cationic, anionic or non-ionic charges. They are normally very effective flocculants agents when added in liquid form at low dosage rates.
Clay-based flocculants are the newest form of combined technology. They are a blend of metal salts, polyelectrolytes and highly active bentonite clays. This type of flocculant is added to wastewater in a dry form and creates multiple reactions when wetted.
Each of the technologies mentioned has its advantages and disadvantages. Determining the type of system that would be most beneficial for a particular application requires a detailed analysis of the wastewater, physical plant, local regulations and requirements of the operation.
Most wastewaters have suspended solids in them. These solids are made up of dirt, metal particles, and other contaminants. Metals in water can also be soluble and not in a particulate form. If separation of suspended solids is the only treatment necessary, most of the mechanical treatment systems previously mentioned will be effective.
Much wastewater comes from cleaning operations, and most cleaning is accomplished with some type of soap. As a result, the majority of wastewaters from cleaning operations contain surfactants. Surfactants work well in cleaning formulations because they tend to emulsify any residual oils, lubricants or coolants that are on the parts being washed.
But cleaning operations also result in the creation of complex chemistries in the wastewater. The combination of emulsified oils and soluble metals makes it hard for any single piece of equipment or chemical to separate the contaminants from the wastewater. Mechanical systems cannot break the complex chemistries in cleaning waste streams, so the only viable treatment for these wastewaters is a chemical system.
Because oils will break out of emulsion at low pH levels and metals tend to precipitate out of water at higher pH levels, initial pH adjustment is beneficial. Because oils and metals react at different pH levels, separate systems for removal of oils and metals are needed. This tends to require many tanks and large amounts of floor space. Multiple tanks are needed to isolate each reaction and avoid having it interfere with another reaction. Tanks necessary for removal of emulsified oils in a standard chemical treatment system include a pH adjustment tank, a polymer flocculation tank, a settling tank and a sludge holding tank.
This system of tanks would need to be duplicated to also treat the wastewater for removal of soluble metals. The polymer chemistry needed for removal of oils is also different than that used to remove metals, necessitating purchase of at least two different polymers for the treatment system.
Water flow through such a system is as follows:
Now devoid of oil, the waste stream is sent through a similar process to remove soluble metals:
The metal-laden flocculant is transferred to a clarifier to settle into a concentrated sludge, collected in a sludge
holding tank and delivered to a filter press or belt. The supernatant water can normally be sent to sewer at this point.
Use of clay-based flocculants can, in many cases, streamline the wastewater treatment process by allowing removal of soluble metals, suspended solids and emulsified oils with a single chemical addition and no pH adjustment. They do this by combining pH adjustment, polyelectrolytes, metal salts and colloidal clays in a single dry chemistry.
Clay-based materials have simplified wastewater treatment by maintaining the dry, blended components in a dormant state until they are added to the wastewater. When added to wastewater and mixed for a few minutes, this dry, blended material packs a powerful punch.
The dry material undergoes a series of chemical reactions as it is mixed and wetted in the wastewater. First, the acid component in the formula solubilizes to reduce pH in the wastewater and break oils out of emulsion. Next, the cationic polymer attracts the free oils onto it. Following this, the alkaline-based component in the formula solubilizes and precipitates any metals in the waste stream. The base component in the formula—sodium bentonite clay—has an overall negative charge, so it attracts the metal particles and coats the cationic polymer.
This starts a reaction that results in the contaminants being permanently affixed onto the clay flocculant. Once the flocculant is dewatered, the sludge will pass the U.S. EPA’s Toxic Characteristic Leach Procedure (TCLP) without no further treatment or conditioning. The treated water at this point meets sewer discharge standards and can be directed to the sewer or recycled to create a closed-loop wastewater treatment system.
So what type of wastewater treatment system is right for your facility? The most important question to be answered before you decide on a treatment system is how to treat the wastewater to comply with regulations at the lowest possible cost.
Wastewater composition and characteristics are an important determining factor in your selection. If your wastewater contains emulsified oil, you can probably rule out using separation equipment including coalescers. No coalescing media or other type of separation equipment can break oil out of emulsion. Instead, oil is emulsified through mechanical mixing or through use of chemicals such as soaps and surfactants.
Filtration systems must also be ruled out in this situation, because they require the wastewater to be filtered through a media which will ultimately become coated with oil and foul. Evaporation is the only mechanical system that can work with emulsified oil, because water will evaporate and thus separate from the oil. Potential problems with evaporation systems include concentrated oil coating the burner tubes and reducing evaporator efficiency.
If your wastewater contains soluble metals, they can normally be precipitated by pH adjustment. But the resulting particles are too small to be removed through separation equipment or filtration equipment other than membrane filtration. Ultrafiltration seems to be capable of removing most metals from water. Evaporation also will remove metals from wastewater and leave a concentrate. Soluble metals can also be removed by treatment with polyelectrolytes and clay-based flocculants as described earlier.
If your waste stream contains suspended solids, they can normally be removed by all of the mechanical wastewater systems mentioned, by polyelectrolytes and by clay-based flocculants. On the other hand, wastewater containing emulsified oils can easily be treated using metal salts, polyelectrolytes and clay-based flocculants.
Your physical facility and the amount of floor space available for wastewater treatment equipment is also an important consideration in determining the type of system applicable. Most mechanical systems require a relatively large amount of floor space because separation of solids from water is accomplished over time. Tank size is based on flow rate, and tanks have to be large enough to allow sufficient time for solids to separate out of the wastewater.
Chemical systems using metal salts or polyelectrolytes also may require large amounts of floor space, but for a different reason. These types of systems, as mentioned earlier, are multi-step treatments requiring several tanks and different chemicals to provide effective treatment.
If sufficient space is not available for wastewater treatment equipment, the most viable option may be a clay-based flocculant system. Since effective treatment is normally achieved with a single dry chemical addition to the wastewater, a “package” wastewater treatment system consisting of a reaction tank mounted directly over a gravity indexing filter bed and clean water collection tank may be the best equipment choice. Such “high-rise” systems takes up minimal floor space.
Other factors in your selection will be based on environmental regulations in your area. This will require investigation of local sewer discharge limits, landfill requirements and wastewater permitting procedures. You may find out that you need to treat for metals that you didn’t realize were in your wastewater. The landfill may require resulting sludge to pass a free water test, or may also require the sludge to pass TCLP testing.
Bench-scale testing of potential wastewater treatment approaches, and detailed analysis from a qualified laboratory, can help you determine how effective a given treatment is for removing contaminants from your wastewater and also how effective the treatment is at tying up the contaminants in the sludge so they don’t leach out into the groundwater.