When working with small particle removal, experience has shown that the addition of chemical coagulants or flocculants is usually a necessity. The question is what is the best method of applying this system?

First, all the applicable terms need to be defined:

1. Clarification: this term can be used to describe the overall process of removing fine particles from water
2. Coagulation: is the act of neutralizing the charges of particles that allow them to repel each other.
3. Flocculation: occurs when neutralized (coagulated) particles are brought together to form larger compounds called ‘flocs’
4. Sedimentation: separating solids from water through settling
5. Creaming/Flotation: separating solids from water by floating them to the surface
6. Dewatering: separating solids from water through filtration

The first stage in the clarification of water is coagulation. When small particles are involved, coagulation usually has to be achieved through the addition of chemical aids. The next stage is flocculation and whether or not another chemical is required here is dependent on the particles and the desired speed of floc formation. The final stage is separating the flocs from the water.

Dissolved Air Flotation, referred to by its acronym of DAF, is a method of removing contaminants from wastewater that have a tendency to float, such as fine solids. Pressurized air is injected into a water stream which is commonly sourced from post-treatment clarified water. That water is mixed with the incoming untreated water where the dissolved air is no longer pressurized and comes out of the solution in tiny bubble form. These bubbles attach to the contaminants and together they rise to the surface. The contaminants can then be skimmed off and disposed. Clarified water exits through the bottom of the tank to ensure the floating contaminants do not continue past the unit.

A progressive passive filter is simply a series of screens with increasingly smaller openings that trap solids as the water flows through the unit (Figure 1). It is designed to be a gravity-fed process and is best suited for low volume flows. The screens are installed on an angle and the water level gradually climbs as the screen becomes clogged. If the screen becomes fully clogged, the washwater can overflow into the next area. The filter can be easily cleaned by removing the screens and washing off the solids. Depending on the settling capability of the solids begin filtered, the tank holding the screens may also need to be rinsed.

Ultrafiltration (UF) is a method of filtration that removes any particles larger than 0.04 microns. The wastewater passes through specially designed membranes under pressure. The membranes capture unwanted material and will need regular cleaning to prevent fouling. Some UF systems will have automatically cleaning cycles that greatly reduce the amount of maintenance required. UF has many applications and may be able to effectively remove muck soil particles and dissolved nutrients from vegetable washwater. The HMGA Water Project is working with Newterra, a Canadian engineering company that specializes in wastewater treatment. Newterra provided a UF pilot unit that was placed after a settling tank system at vegetable washing facility.

Harvested root vegetables are washed prior to packaging for sale. It is a multi-step process to clean the produce. Before the vegetables come into contact with water, the dry soil can be removed which will reduce the amount that must be washed off. Not every packing process uses this step, which is a valuable tool both for removing soil and for reducing the amount of work required to treat the washwater prior to discharge.

Results: Under typical processing conditions this system is able to remove 1 cubic foot of soil from every 2750 pounds of carrots washed for a total of 64.9 cubic feet over one day of washing. This soil removal lessens the solid load in the washwater treatment system. Dry soil is also easier to handle and dispose as compared to wet soil mixed in with water. Over one day of washing, this pre-wash system intercepts 39.3 grams of phosphorus and prevents it from entering the washing system’s wastewater. This is less phosphorus that would need to be removed by treatment prior to discharge.

Conclusion: The addition of a dry soil removal system diminishes the pressure on solids removal treatments and nutrient removal prior to discharge. They can be installed into an existing wash line with minimal modifications as they also act as conveyors or turning mechanisms.

Next Steps: The implementation of this type of system would have a great impact based on the collected data. If all the carrots grown in the Holland Marsh area including surrounding growing vicinities, an average of 44.6 kg of phosphorus would be prevented from entering the watershed.

The washing of root vegetables is a multi-step process. Not every processing and packaging plant has the same system but most are based on similar designs. The steps shown in Figure 1 may not all be present, some may have additional steps, spread over several pieces of equipment, or steps combined into one piece of equipment.

The first task to be completed is removing soil from the vegetables prior to a wet wash. A soil removal belt can jostle dried soil from the vegetable and reduce the amount of soil that will need to be subsequently washed and treated. The initial wash is usually done in a batch tank where the vegetables are dumped into water. This can also double as a flume to move them to the next step in the process. The water discharged from this step will require the most amount of treatment as it will have the highest concentration of soil.

Following initial washing, a polisher is used to do a final clean on the vegetables. This is done with a combination of water, brushes, and rollers to polish the roots. The last of the soil will be removed at this point. Nutrients and organic compounds from the vegetative material will also enter the water discharge stream due to the polishing.

In some facilities the vegetables will be peeled and cut. The water used in this process will have nutrients and organic (soluble) compounds when discharged.

The last stage before packaging is a final wash with potable water. This is required for food safety and the water discharged will carry little waste as the produce has been thoroughly cleaned in the previous steps. This water can also be re-used.

The amount of solids and nutrients in discharge water will depend on several factors. If a dry soil removal step is not present, all the soil will need to be washed off into the water, increasing the amount of solids in the discharge. There are also opportunities to reuse water from later stages in the preliminary steps; for example, the discharged water from a final wash can be used in the polisher and afterwards be added to the batch tank. In this way water is used more than once and a higher concentrated lower volume discharge is entering a treatment system.

For more information on the drum filter test, see Drum Filter Demonstration Site.

Filtration systems are not always a simple installation. Technologies require some manipulation to ensure that they are functioning at their maximum efficiency. The drum filter that was installation is one example. It succeeded in taking in washwater and filtering it through the screen. It was successful in rotating the barrel and spraying off the waste into a collection tray. However, initially the output of the collection tray contained excessive amounts of water. As the goal is to have a sludge-like material with the least amount of water possible to limit amounts of waste to dispose, adjustments were in order. The drum filter is able automated to rotate and spray on a regular schedule; the purpose of the optimization process is to find the setting that produces the most concentrated waste stream.

Flow monitoring is an important component of a water management plan. It is useful to know the volume of water that is used while washing and processing as well as the volume that is discharged as waste. If the flows within the processing and washing facilities are monitored it may be possible to identify inefficiencies or opportunities where less water could be used. A complete understanding of water use within the plant can provide new information that can help with implementing recycling and reuse of washwater.

If discharging is unavoidable flow monitoring is still very important because when considering wastewater treatment it is crucial to know what volumes will require treatment. The less water the better, and generally less expensive. Volumes are needed, along with pollutant concentrations, to calculate the loading of waste into a treatment system. This information is also vital for selecting a water treatment technology that will be suited to treat the washwater.

There are several ways to monitor flow. A simple option is to use the flow rate of a pump and the amount of time it runs to estimate flows. Flow meters are an accurate and easy way to monitor flow, but they can be expensive. However, when a complete water management plan is being developed it is worthwhile to invest in proper flow monitoring equipment. By understanding the volumes of water used on a farm finding ways to decrease water-use and treat washwater becomes much easier.