Some solids are too small to be captured by solid removal techniques. They need to be compounded together in order to make them large enough to be efficiently separated from water. This process is referred to as coagulation and flocculation.

​Electrocoagulation is a method of coagulation and flocculation which applies an electrical charge to make the particles come together by changing the charge on the particles’ surfaces. The wastewater is held in an area where an electrical field is created using charged metal surfaces called anodes and cathodes. Applying the charge to the water destabilizes the bonds holding compounds such as solids and nutrients to water molecules. It will also strip the charge from colloids. These destabilized particles now come together to form a mass. This process will create a stable floc of particles that rises to the surface, a sludge layer on the bottom, and clarified water in the middle. The waste can be removed by skimming from the surface and collecting from the bottom; it can then be dewatered and composted. The clarified water is removed from the center and brought to its next destination.

The alternative to electrocoagulation is using chemicals to coagulate and flocculate. The advantage of electrocoagulation is the waste stream is free of any added compounds. Chemical dosing systems are also greatly dependent on the pH, composition of the water to be treated, and the flow rate. The dose may need to change regularly in response to these factors in order to be fully effective. The disadvantage of electrocoagulation is that it is reliant on a sufficient electrical supply and will require a sludge de-watering system. Both versions of coagulation require on-going inputs, whether be chemicals or electricity.

​Problem: A greenhouse facility wants to reuse water from a flood floor which is used to water potted plants. The water collected has vermiculite and organic matter clouding the water which would clog nozzles and settle in pipes.
 
Solution: The operator looked at two different technologies, a self-indexing filter and a parabolic filter screen (Figures 1 & 2). Both are recommended units that filter coarse material from water.

​Discussion: The infrastructure required for either technology is similar; both will need some plumbing and a pump to move water through the treatment system. The technologies are compared in the table below. Both systems will complete the task; the self-indexing filter is more hands-off but requires more inputs. The parabolic filter screen must be supervised but has minimal on-going costs.

​Decision: The operator choses the self-indexing filter as it requires less regular supervision. The additional energy costs, on-going need of paper rolls, and disposal of paper was preferable to increased labour requirements and disposal of waste.
 
Lesson Learned: When there are multiple technologies that could fit in a treatment system it is important to investigate all aspects. Decisions should be made based on more factors than merely cost.

The HMGA Water Project evaluated coagulation and flocculation systems for removing fine solids from washwater. They were used in conjunction with large solid removal technologies and compared to other systems with no added chemicals. A summary of the tests and results are available in the article below.

This project has produced an array of factsheets on a range of topics. Below is the list grouped into topics and arranged in a suggested reading order. The documents can be found here, unless otherwise noted.
 
Water Qualities
#012 Water Quality Standards in Agriculture
#001 Water Quality Parameters for Vegetable Washwaters
 
Regulatory Considerations
#004 Considerations when Determining Discharge Limits
#018 Selecting a Laboratory
#006 Water Sampling & Proper Procedures
#009 Regulatory Permitting & Compliance
 
Technology Considerations
#016 Design Considerations for Vegetable Washwater Treatment Systems
#007 Choosing Washwater or Water Treatment Technologies
#002 Impact of Muck Soils on Water Treatment Systems
 
Large Solid Removal
#014 Large Solid Removal for Effective Treatment
Technology Investigation: Filter Bags (available here)
#005 Settling Ponds & Tanks
#008 Drum Filters
#010 Hydrocyclones & Centrifuges
 
Small Solid and Nutrient Removal
#003 Coagulation & Flocculation
#013 Biofiltration
Technology Investigation: Ultrafiltration & Capacitive Deionization (available here)

Polishing and Dosing
#011 Bottom Aeration
#017 Surface Aeration
#015 Water Treatment Technology Options for Washing Vegetables

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.

A drum filter was placed at a washing facility to test its performance. Drum filters function by allowing washwater to fall through a circular metal screen. As the screen becomes plugged by solids, the water level rises until it trips a sensor. This signals the machine to rotate the drum and turn on a spray bar which cleans the waste off the screen into a collection tray that exits through an outlet. The water used to rinse is pumped out of the lower portion where filtered water is collected prior to flowing out. It can also be programmed to rotate and rinse on a set schedule.

The filter tested is rated to handle 1,000 US gallons/minute with a solid load of 10 mg/L using a 30 micron screen. However, the washwater it was treating had a lower flow rate and a higher solid load. It was installed after a barrel washer for root vegetables and before a biofiltration system intended for dissolved solids and nutrients. Theoretically this drum filter would remove suspended solids larger than 30 microns in size, leaving the finer solids and dissolved portion for the next treatment step. 

HMGA Water Project recently collaborated with Bishop Water Technologies to test a Geotube® at a vegetable washing facility. The purpose of the pilot project was to demonstrate the feasibility of this technology under this type of system. The process leading up to the on-site test and the results found are available in the article below.