Insights into Wastewater System Components

Wastewater treatment systems for metal finishing can take many shapes, depending on a facility’s need. All wastewater systems, no matter the size, are designed to remove contaminants so the effluent can be discharged to a receiving body, or reclaimed and reused internally for production purposes.

A wastewater treatment system is made up of several crucial components, including one or more tanks, mixers within the tanks, pumps, lamella clarifiers, a dewatering system, pH and sometimes oxidation reduction potential (ORP) meters. Each of these elements’ proper functionality is critical and necessitates maintenance to keep the wastewater system running smoothly. When piecing together the most effective wastewater treatment system for an operation, it is important to understand how each of these components work and the options available for each.

Tank types

A wastewater tank is the basis of all systems. Without it, wastewater has nowhere to go as there is no contaminant and no vessel in which to treat the wastewater.

The tank is usually made from either polyethylene or fiberglass. Polyethylene tanks last about 10 to 15 years, while fiberglass tanks last between 30 to 40 years. Fiberglass has increased rigidity for more structural support. However, these tanks can be more expensive than the polyethylene tanks.

Choosing the correct tank for a system will depend on some factors. Will the system be a batch system? If so, a conical bottom will enable the tank to act as an all-in-one treatment vessel. The pH adjustment, chemical additions, flocculation and settling can be done in the one conical bottom tank. When the solids are fully settled, they can easily be removed from the cone for dewatering and disposal.

However, with a flow-through system, solids should not be settling out in the tanks; therefore, flat-bottom tanks make more sense in this case. The volume of wastewater that will flow through the tank, if the wastewater will be hot, and the types of contaminants that will be present in the wastewater are other considerations.

Mixing it up

Mixer placement in a tank, along with speed and rotation of the blades, is critical to ensure it is doing the job properly. With proper placement, a mixer enables the wastewater to come in contact with the necessary chemicals, moves the water along a desired path in a tank and ensures that solids do not settle. Rotational direction and speed can assist in floc formation or cause the floc to break down and not form properly. If the speed is too fast, it can cause air to become entrenched in the wastewater. If this happens, you can see floating floc in the clarifier as the air gases off to return to the atmosphere.

Because mixers move wastewater along a specific path in a tank, short-circuiting does not occur, which happens when the flow of water through a tank is directly through the tank. But when it does happen, the wastewater treatment process is shortened and chemical contact is reduced or, in some

Integrating the correct pump size is crucial to ensure that they last their expected life cycle.

cases, eliminated from the tank. Baffles can be installed inside to prevent short-circuiting. Baffles are typically 6 to 8 inches shorter than the wall of a tank and extend 8 to 12 inches into the tank.

Moving wastewater between tanks, adding chemicals for treatment and moving the solids is accomplished with either gravity drainage systems or mechanical means. Gravity drainage systems have pipes installed between tanks at such an angle that water uses gravity to move between the tanks.

Under the pump

When gravity will not work, pumps are installed, which can be air driven or electrical. Through pipes, pumps move either the wastewater, the chemicals to treat the wastewater or the solids produced from the treatment of the wastewater from one location to the next. In metal finishing, most pumps are diaphragm, centrifugal or peristaltic.

Integrating the correct pump size is crucial to ensure they last their expected life cycle. A pump has a maximum capacity based on how much it could move in a 24-hour period if maxed out. This is not an ideal state of operation and is something that pump manufacturers do not recommend. Instead, it is advised to calculate how much the pump needs to flow and implement a pump that is one size larger.

For example, if you need to move 60 gallons an hour of wastewater, a pump should be installed that can flow 100 gallons an hour. The pump can then run at slightly more than half capacity and will last longer. The reverse is true as well. If 60 gallons an hour needs to flow and the pump can move 600 gallons an hour, it is oversized. It may struggle to move the smaller amount and stall out regularly. Pump care is also important to ensure that it lasts the duration of its expected life cycle. Routinely inspecting the pumps for leaks, repairing leaks as soon as they are found and cleaning pumps by flushing with water when the system will be shut down for extended periods of time are all part of pump care.

Clarifying the issue

Most metal finishing facilities use a lamella, or incline plate clarifier. While there are other types of solid/water separation units (such as dissolved air flotation or a round clarifier), for the purpose of this article we will focus on the lamella style. Lamella clarifiers enable the solids to settle to the bottom while the clean effluent flows out the top. The lamella’s design enables the solids to slide down the plates that are placed at a specific angle.

Routine care of a lamella clarifier includes totally draining the clarifier and hosing down the walls and plates on a routine basis. The plates should be pulled out, inspected and deep cleaned to remove scale on an annual basis. Doing so can help ensure the 15- to 20-year life expectancy is met.

Dewatering system

Once the wastewater has gone through the entire treatment process, the settled sludge needs to move to a dewatering system. Dewatering systems can have a sludge thickener tank or can go directly from the settling point to the dewatering equipment. A sludge thickener tank enables the solids to continue settling and separating from the wastewater, enabling an easier dewatering process. Dewatering equipment can be a plate and frame press, a rotary vacuum filter, a screw press or a belt press, to name a few.

Plate and frame presses are the go-to for most metal finishers. The plate and frame press is designed to dewater a certain amount of sludge based on the desired press cycle time, the amount of sludge that needs to be dewatered and the plans for future growth. Routine care of the plate and frame filter press includes cleaning the plates to ensure they do not become blinded in the next pressing, and replacing the clothes as needed. When a plate becomes blinded, the press cannot move the wastewater through and it becomes trapped in the plates. This will give a false full reading, so when the press is opened, the wastewater and sludge comes out.

Cleaning the cloths can be done with an acid wash system, manual cleaning or by sending the entire plate with cloth to an outside company for cleaning. The single most important thing for returning the plates to the press is ensuring they are placed back in proper order. If they are out of order, the wastewater cannot move through the press properly, resulting in poor cake formation.

Tests to remove metals

Besides the tanks, the next most important component of a wastewater system is pH and ORP meters. While not all systems will need the capability to read ORP, they all need to use pH because it is the basis of removing metals from the wastewater. Both hydroxide and sulfide precipitation are pH dependent. Placement of the probes in the tanks is critical. If the probe is in a spot where it does not encounter a fully mixed solution, it will give a false reading. Probes should be pulled from the tanks on a regular schedule to be cleaned and calibrated. Solids can build up on the probes, blinding them and causing false readings. The tip of the probe can become scratched or damaged when in use. In a perfect world, the probes would be pulled daily for visual inspection, cleaning and calibration. 

Replacing or upgrading

When deciding whether it is time to replace the current wastewater system or upgrade an existing system, some questions to ask include:

• Is the current equipment in good shape or is it damaged from years of abuse?
• Is the current system keeping up with the demands that are placed on it or are tanks overflowing because the throughput is more than it was designed to handle?
• Is there space to install a new system so the old one can keep running until the new one is brought online?

Wastewater treatment does not need to be complicated, but it is critical to get it right. A system that is in poor condition can, in the long run, cost more than replacing it.