Michigan Utilities Choose Co-Precipitation Arsenic Removal

by Rick Mann 
Tonka Equipment Company

There are several Best Available Technologies (BAT) for arsenic removal from potable water, and after careful consideration of the advantages and disadvantages of each, many Michigan water utilities have chosen Oxidation/Filtration, specifically in the form of Iron Co-Precipitation Filtration. Recently, Tonka has provided systems for Michigan communities that treat over 50 million gallons of water per day.

Arsenic Removal in Holly, MI

Effective, Efficient Treatment for Arsenic Removal
Iron co-precipitation filtration is a simple and very cost-effective method for treating arsenic, and the technology also removes iron and manganese. There are four steps:

1. Oxidation - Soluble iron, Fe(II), and arsenic, As(III), are oxidized, typically with chlorine. This step forms iron hydroxide solids and As(V) arsenate.
2. Co-Precipitation or adsorption – As(V) arsenate adsorbs or coprecipitates with the iron hydroxide solids. Additional iron may be added as Fe(Cl)3.
3. Filtration – Particles/precipitants (ferric arsenate) are filtered from the water.
4. Simul-WashTM Backwash – Particles/ precipitants are backwashed to waste.

The oxidation step creates a form of arsenic As(V) arsenate which is preferred for iron co-precipitation because it is less toxic and less soluble. Arsenate is a charged anion, which makes it easier to remove with Iron Co-Precipitation.

Arsenic removal efficiency is directly related to iron concentration.  Tonka has had excellent removal at all of our plants in Michigan using naturally occurring iron. In the rare occasion (in MI) that the natural iron to arsenic ratio is not sufficient, additional iron can be added to achieve
optimized arsenic removal.

Pressure or Gravity Vessels
Co-precipitation treatment can be offered in either gravity or pressure vessels with excellent performance.

Pilot Study
Piloting is a key step to demonstrate what level of arsenic removal can be obtained and to determine if iron levels are adequate or if additional chemical feed is required.

Backwash
The backwash step removes As/ Fe(OH)3 solids from the filter. Arsenic does not build up in the filter media and there are no solid-hazardous waste disposal concerns as may occur with accumulative medias. Tonka’s Simul-WashTM backwash reduces the backwash volume by 50% over all other backwash methods.


Michigan Water Plants removing arsenic with co-precipitation(Total MGD: 53.7)

• Charter Township of Oxford (Granger Water Plant, Oxford Woods)
• Independence Township (Deer Valley, Deerwood, Deerwood Manor, Lake Oakland Woods)
• Holly 
• Mayville 
• Millington 
• Centennial Farms
• Mattawan 
• Caro 
• Carsonville
• Pentwater 
• Yankee Springs 
• Cass City

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