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Clarifying the Solid Removal Process

3/28/2016

4 Comments

 
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Figure 1: Examples of creaming in settling tanks
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.
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Figure 2: Coagulation, flocculation, creaming, sedimentation, and dewatering
Floc separation can occur three different ways: sedimentation, creaming, or dewatering. The method chosen will depend on the type of flocs as some will naturally sink and others will prefer to float. It is important to match the coagulant and flocculant chemicals, floc behaviour, and method of separation to make a clarification system operate properly and efficiently.
Each separation method has its own challenges including infrastructure needs. Sedimentation is most commonly completed using a settling tank. The separated solids will need to be scooped out regularly so the tank has sufficient room to collect solids. Creaming is associated with air flotation where solids either naturally float or are aided with dissolved air carrying solids upwards. The solids are then skimmed off the water surface. Dewatering uses filters to collect the solids behind a membrane. The membrane will require regular cleaning or replacement when the solids clog the pores. Water used to clean the membranes will also need to be considered for further treatment.
References
  • Tramfloc, Inc. (2014). Selecting Polymers, Jar Testing Procedures. In Flocculants. Retrieved March 14, 2016, from http://tramfloc.com/polymers-selection-jar-testing-procedures/
  • GE Power & Water. (2012). Chapter 05 - Clarification. In Handbook of Industrial Water Treatment. Retrieved March 14, 2016, from http://www.gewater.com/handbook/ext_treatment/ch_5_clarification.jsp
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Dissolved Air Flotation

3/14/2016

1 Comment

 
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.
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Figure 1: Diagram of a DAF unit (Komline-Sanderson, 2015)
The solids to be removed may be too fine to be caught by the rising bubbles. In these cases, coagulants and/or flocculants are used to aggregate them into larger clusters. The coagulants and flocculants are added either in a preceding tank or piping system.
​
DAF systems will have a smaller footprint than a settling system as there is more active movement in a DAF system. It is an indoor system as it should not freeze. The timing of any chemical additions for aggregation of soils must be carefully considered as it will take time to properly bind the solids. If they are added too close to the DAF, full flocculation will not occur and that chemical will be wasted.

​References
  • Mundi, G. S. (2013). Assessment of Effective Solids Removal Technologies to Determine Potential for Vegetable Washwater Reuse (Master's thesis). NovemberRetrieved from https://atrium.lib.uoguelph.ca/xmlui/bitstream/handle/10214/7737/Mundi_Gurvinder_201312_Msc.pdf?sequence=3
  • Komline-Sanderson. (2015). Applications. In Dissolved air flotation. Retrieved December 14, 2015, from http://www.komline.com/images/tab_DAF_app.png
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Progressive Passive Filtration

2/29/2016

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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.
This system is intended for low volume washing facilities with low solid loads. It will easily handle inconsistent flows as long as the flows don’t overwhelm the filters; it should be sized for the maximum flow or the flow be restricted to the filter’s capability. The number and sizes of the filters are chosen based on the type of solids to be removed. The final filter should be 100 microns as solids that pass through will not settle in pipes and block water flow.
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Figure 1: Diagram of a progressive passive filter
The advantage of this type of system is that it is a compilation of parts that can be inexpensively sourced. It can be built to any size necessary and be fitted into an existing tank system. The disadvantage is that it is not useful in a large-scale production and it does require manual operation and maintenance.
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Figure 2: Side view (far left), top view (center left), inside view (center right), and the outlet and 100 micron screen (far right) of the progressive passive filter constructed based on the diagram of Figure 1.
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News Release "Technology Investigation: Ultrafiltration & Capacitive Deionization"

1/25/2016

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HMGA Water Project collaborated with two technology providers to test the feasibility of their systems at treating vegetable washwater. An ultrafiltration unit from Newterra was followed by capacitive deionization from Voltea at this on-site pilot project. A summary of the test and results can be found in the article below.
Technology Investigation: UF & CapDI
File Size: 802 kb
File Type: pdf
Download File

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Ultrafiltration & Deionization Demonstration Site

12/14/2015

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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.
Capacitive deionization (CapDI) is an effective technique used for the removal of dissolved ions from water. CapDI uses negatively or positively charged electrodes to attract and hold ions, which also have electrical charges. Negatively charged electrodes attract positive ions and the positive electrodes attract negative ions. Each CapDI system can be designed to target different ions depending on the chemistry of the wastewater. CapDI is commonly used for the removal of salt from water (desalinization) but it can also be used to target dissolved nutrients commonly found in vegetable washwater. The HMGA Water Project partnered with a Dutch company called Voltea to run an on-farm pilot test of a CapDI system.

The ultrafiltration system was placed before the CapDI system as it is able to remove the fine solid particles that would foul the CapDI (Figure 1). There were several additional water lines and a holding tank required to complete the test set up.

​Results to follow
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Figure 1: Organization of test including both technologies and water flow path
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Dry Soil Removal

11/23/2015

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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.
Goal: Decrease the amount of soil entering a washwater treatment system.

Solution:
 Install a dry soil removal system prior to vegetables entering the washing process. The vegetables are sent over a finger table (Figure 1) that jostles dried soil off. This system is also able to be used in the field on a harvester. Then they travel on a belt towards an angled belt where the vegetables roll down due to gravity and the angled belt, travelling in the opposite direction to the vegetables, carries soil or dried leaves away (Figure 2). The majority of the soil is removed by the finger table.
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Figure 1: Finger table; this system can be used in place of a conveyor belt or to turn the direction of vegetable flow
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Figure 2: A generalized version of a soil removal belt (A & B) and one integrated into a wash line (C)
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.
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Vegetable Washing Process

10/26/2015

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Figure 1: A generalized vegetable washing, processing, and packing process
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.
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Polders & the Holland Marsh

9/8/2015

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A polder is an area of low-lying land that has been reclaimed from a body of water and is protected by dyke and pump systems. Once the land to be reclaimed has been selected, dykes are built around the area. The water within the dykes is then pumped out of the area into a system of constructed canals (Figure 1). The land remaining is below the natural water level and pumps are used to continually remove water from the area. The polder is physically separate from the surrounding water system as the inflow and outflow are controlled through pumping stations. Natural seepage from ground water also contributes to maintaining soil moisture.
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Figure 1: Side view of a bog (top) turned into a polder (bottom)
The Dutch are well-known for their polders. They have reclaimed sea beds using this technique, holding back the sea with large dyke systems and pumps that were once powered by windmills. Other polders can be built to reclaim land from lakes, rivers, or bogs. Rich, fertile soils reside within these polders; once drained, polders have clay or muck soils, or a combination of the two. The land is largely flat and treeless, which is very suited to agriculture. 
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Figure 2: Overhead view of the northern end of Holland Marsh where the canals converge with the Holland River at the pumphouse [date unknown]
Draining the Holland Marsh, named for the first Surveyor General of Upper Canada Major Samuel Holland, was a project led by Professor William Day of the Ontario Agricultural College beginning in 1925. The 28 km of canals dug then now protects 7,000 acres of prime muck soil. Pumping stations, one situated at the North Branch and the other at the northern end keep the water level low enough to keep the land dry. There is an additional 2,500 acres in smaller polders along the Holland River as it runs north to Lake Simcoe.
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Lesson Learned: Drum Filter Optimization

8/4/2015

2 Comments

 
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.
Table 1: The time between each spray cycle, approximate volume of waste output, volume of waste output per minute, and solids (depth in sample bottles) for each spray cycle setting
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The first step was to investigate the settings that were most appropriate for the solid load and flow rate of the washwater. The drum filter was run at each of those settings for a few cycles and sludge samples were taken along with waste output volumes, spray cycle time, and time between rotations was recorded (Table 1). The samples were left to settle so the solids portion of the sludge could be observed (Figure 1).
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Figure 1: Illustration of waste output at a spray cycle setting of (left to right) 5 seconds, 10 seconds, 15 seconds, 20 seconds, and 3 samples at 25 seconds.
Based on the results outlined in Table 1, the samples taken from the 5 second and 15 second rotation had the highest amount of solids in the sludge. The 5 second spray cycle has the lowest overall waste output. It was determined that this setting was the most efficient out of those tested.
Prior to optimizing the cycle setting, the change in total suspended solids between pre and post drum samples was 46%. The drum filter was run using the 5 second spray cycle setting and samples of the pre drum, post drum, and sludge were taken. After changing the setting, the change pre and post drum was 71%. Through manipulating the spray cycle, the efficiency of the system was greatly increased.
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Figure 2: Total suspended solids in pre drum, post drum, and sludge samples prior to optimization on day 1 and after on day 2
Lesson Learned: As with many technologies, it takes time and careful measurements to set equipment to operate at optimal levels.
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Flow Monitoring

7/6/2015

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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.
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Figure 1: Flow meter and logger unit (left), flow meter attached to a circular pipe band (center), and installed flow meter (right)
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.
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Figure 2: An example graph showing data collected by a flow meter
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.
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