I have found DTC to be a very expensive proposition in the long run 
just to get the last few ppm of copper or lead out of a waste stream. 
 The problem usually has been a little bit of hydroxide fluf  floating out 
off the system .   Usually a  final filter will capture all of the 
material and get you within limits very easily and automatically 
(even at 150 gpm).

Check out what the cost of changing for a year would be, then check 
out the cost of an automatic final filter to polish the waste stream. 
  I'd bet you could add a duplex final filter and a chelated metal recovery 
system at the end for what the cost of the mag/hydrox and DTC over 2 
years.  

You could also investigate changing polymers.  Just because the 
polymer you are using doesn't work with stripper/surfactant solutions 
doesn't mean you are dead.  You may even be able to use a blend of 
polymers to get you into compliance easily.  

Another problem may be flow rate.  Your sludger may be designed for 
1/3 of the flow you're running through it.  You may find that a 
little water conservation in front of the system will help you alot 
out back.  I've seen shops cut water flows by 60-70% with a little 
thought.

The real problem is with using a precipitation system in the first 
place because they are very hard to automate.  They are sensitive to 
variations in flow rates an to sludge volumns.  You have to watch 
them pretty much constantly.  You can keep band- aiding  a sludger forever.  
I don't actually know of  a PC shop buying  a sludging system in the last 5 years.

Bob Mesick
Remco Engineering
www.remco.com/home.htm

> Date: Fri, 14 Feb 1997 22:54:59 +0000
> From: Paul Gould <[log in to unmask]>
> To: [log in to unmask]
> Subject: Re: mag hydroxide in waste treatment -Reply
> Message-ID: <[log in to unmask]>
> 
> In message Tat Choo  writes
> >Basically the magnesium hydroxide (mixed in a tank with mixer, or
> >supplied in a slurry form) is meter into the settling tank that
> >contains the pre-reacted metal bearing DTC floc, and the sludge is
> >supposed to settle much faster than if using DTC alone. The process
> >is also commonly used with waste water having high concentration of
> >organic such as stripper/developer or surfactant bearing cleaner
> >solutions. High organic create sludge that is difficult to settle.
> >The magnesium hydroxide also help to increase the density in the
> >sludge and create a sludge that dewatered faster. This decreases
> >the chance of getting wet cakes from filter press.
> We use Lime or Calcium Hydroxide in slurry form to neutralise to ph
> 8.5 after acid cracking and before adding floc. The lime aids
> settling considerably and improved our treatment 100% when we
> switched from using Caustic solution. I don't know how much
> Magnesium Hydroxide is but it sounds more expensive than lime.
> Regards Paul Gould

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