Slowsand Filter on Groundwater source 1

[Tanisdad]

I recently reviewed a set of plans for a slowsand filter. The source water is ground water and is not under the influence of surface water. I am concerned with the use of slowsand with groundwater. I do not believe that a schmutzdecke will form. Has anyone heard of using slowsand on groundwater. What concerns would you have. Thanks for any information you can provide.

Confused reviewer

 

[bimr]

What are they proposing to accomplish with the filter?

Presumably, groundwater is acceptable as is without filtration. If the groundwater contains suspended matter, then the well should be repaired or replaced.

 

[Tanisdad]

There are two wells with gross alpha issues. The community issued a contract for instalation of an R.O. unit to reduce the amount of Gross Alpha. The R.O. supplier conducted a short pilot study and has stated that a a black biomat is forming on the membranes. He has reccomended to the community that they proceed with installing the slow sand filter to remove the biomat. I have not been able to obtain the pilot data or actually see the biomat. Currently samples or being collected to get a better understanding of the water quality.

 

[bimr]

The first thing to do is get a complete water analysis.

A few observations. Pretreatment has to be considered very carefully when using RO. There are all sorts of things that will foul RO membranes.

Without knowing anything further, I would guess that the black material is oxidized sulfur. If it is sulfur, then the usual course of water treatment is an aearation step to oxidize the sulfur, chlorination to oxidize the remaining traces of sulfur, and then filtration.

You may want to suggest an alternatives to RO. Water softeners are commonly to remove radon for example. Not sure that an RO system is necessary. Whatever is in the water analysis will be the deciding factor.

 

[sbush]

Your concerns about applying slow sand filtration to ground water are correct. You need to look at another approach because slow sand is not going to work. Adsorption clarification/filtration, microfiltration, diatomaceous earth filtration...these are a few alternatives.

Bimr raises a valid question: Why choose RO for gross alpha reduction? RO is typically viable when TDS reduction is required in combination with high gross alpha levels.

High gross alpha readings are typically caused by either radium or uranium isotopes (but not radon, which is a volatile gas). If radium is the source, then a simple cation exchange resin system operating in the sodium form is highly effective. If uranium isotopes are the source, then a simple anion exchange system operating in the chloride form is highly effective. If the source is a combination of both, then cation followed by anion will work.

I agree with Bimr. You need to look at the total water chemistry and weigh all of the options. Try to get outside advice from someone other than the RO system vendor. Your state's drinking water approval authority should be able to comment on technologies and experiences at similar sites.

S. Bush

http://www.water-eg.com

 

[Tanisdad]

I would love to be able to avoid using the R.O. however it has already been purchased and installed. It is not plumbed into the system. I work with the Environment Department and am working hand and hand with the Drinking Water Bureau. The mess has been made and we are trying to provide the best solution at this point. Thanks For the advice from both of you.

Tanisdad

 

[sbush]

Okay. So the community has purchased an RO having inadequate pretreatment for fouling. The vendor is telling you that the fouling is caused by a “black biomat”, which is tech speak for a “black coating that tests positive for bacteria”. Now, just because the black coating and live bacteria appear in the same sample, this doesn’t necessarily mean that they are one and the same, as the term “biomat” infers. Bacteria on membrane surfaces is common and even expected to occur since there are no disinfectants present and contaminants in the feedwater are a good food source. What is not common is the black coating. You need to find out what this is. There are several companies who offer membrane analysis services to determine the types of fouling. B. F. Goodrich, King Lee Chemicals, and Argo Scientific are a few of these companies, and a search of the Internet may return others.

Some possible sources for “black” fouling are: metal oxides such as manganese oxides, sulfate reducing bacteria, and naturally occurring organic matter which can be gray to black in color.

You cannot proceed with a pretreatment scheme, until you know precisely what you are dealing with. Once you have identified the contaminants, then take your problem to an experienced professional engineer. Since the RO vendor has not exhibited these qualifications, thus far, I suggest you look elsewhere. There are several consulting firms in the U. S. who specialize in membrane-based water treatment. Peter S. Cartright is one of these firms that has been around for 25+ years. Although I have not used them personally, I believe they could help you. A search of the Internet may turn up others as well. The key to unbiased assistance is to find an independent consultant who is not an equipment vendor.

S. Bush

http://www.water-eg.com