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Remove excess reactive silica from wastewater


Q. I am chemical engineer with 12 years experience in design and commissioning waste water treatment systems and RO facilities.

I am currently employed as the design head of an ISO 9000 organization involved in the same activity, in India.

We have a process of waste recycle from textile processing, involving zero discharge. The process consists of primary, secondary and tertiary treatment with UF and RO.


RO = reverse osmosis
UF = ultrafiltration

I have a problem with my silica in feed. Typically, silica is in the level of 100 to 150 ppm and MOST OF IT IS REACTIVE SILICA. I have very little colloidal silica.

My question is: By using lime soda softening (hot), coupled with aluminium / zinc dosing, CAN I REMOVAL REACTIVE SILICA?

Needless to say, the high silica is posing problems in the RO plant as well as the downstream mechanical evaporator for evaporating the RO rejects.

Your feedback is welcome.

Venkatasubramanian Rajesh
DESIGN HEAD - Chennai, Tamilnadu, India


A. Dear Mr. Rajesh,

I am sure you may be aware of the following so take it as a refresher only!

It is indeed possible to reduce reactive Silica by precipitation.There are two ways of doing it,

1. By addition of Sodium Aluminate
2. By Addition of Dolomitic Lime.

Of course you need to add Lime or Caustic as this precipitation takes place at 11.5 pH and you get added advantage of partially softened water. The downside is huge amount of lime sludge which you need to tackle! Also if clarification, filtration system is not properly designed you end up having trouble with SDI, which you may face since you mention this as treated textile effluent.


SDI = silt density index

Hope this was useful!

Best Regards

Umesh Sontakke
Water Treatment - Riyadh, Saudi Arabia


Q. Thanks for the response.

Now, in one case, the silica level is 500 ppm for a water TDS of 2500 ppm, with hardness in the range of 50 - 100 ppm as CaCO3.

Can I still do lime soda softening? My flow rate is about 15 m3/hr

Please reply.


TDS = total dissolved solids

Venkatasubramanian Rajesh [returning]
- Chennai, Tamilnadu, India

A. I have been investigating the use of well water to supplement our city water supply for process cooling. The city water is brackish with moderate silica. Unfortunately, the well water is also brackish, with even higher silica, but not as high as you are reporting.

What I found: activated alumina (alkaline wash, acid wash) with approximately 15-30 minutes contact time extremely good reduction of the silica. However, the process forms a suspension of tiny particles (presumably magnesium silicate, since magnesium also slightly reduced by this) that are hard to filter. Coagulants and flocculants have a positive effect, allowing the matter to be clarified.

James Stewart
power & light - Lubbock, Texas
April 19, 2010

A. Dear Rajesh

As you say that your silica is reactive silica, hardness and 2500 TDS then you can use the Ion Exchange Resin to solve total problem.

This is also cost effective in terms of Manpower and electricity cost.


Vishal Parekh
Environmental consultant - Ahmedabad, Gujarat India
November 25, 2010

Q. I need information about clarification purpose. Why do we need to use dolamite and lime instead of ferric chloride [on eBay or Amazon] in HRSCC? What is the mechanism of them?

Please help me.


kranthi kumar
- Hyderabad, telangana, India
August 27, 2015


HRSCC = high rate solids contact clarifier

A. Hi Kranthi. I am not sure if you fully understood what you have read here. It does not say that -- in general -- you should replace ferric chloride [on eBay or Amazon]. ferric chloride[on eBay or Amazon]is a useful coagulant/co-precipitant to aid in solids removal and for water clarity in a clarifier.

This dialog is saying that treatment with dolamitic lime at high pH is a way to deal with excess dissolved silica.


pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey
August 2015

A. Hello - I found this article that may help you. It seems directly on point for your situation.

Aqueous Silica and Silica Polymerisation | IntechOpen

Regards- Aaron

Aaron Sepanski
- Des Moines, Iowa USA
October 9, 2019

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