Heavy Metals & Fluoride Removal – Geebung, QLD
INTRODUCTION – THE PROBLEM
Pacific Water Technology recently commissioned a heavy metal & fluoride removal treatment process for removal of high levels of heavy metals & fluoride out of wastewater coming from an Acid Pickling Plant in Geebung.
The acid pickling plant uses an acid bath to pickle & passivate stainless steel, a manual pressure clean is then performed to clean any residual acid off the Stainless Steel and this rinse water is the wastewater that needs to be treated to remove heavy metals & fluoride before it could be cleared to be discharged into drain.
The wastewater had a pH level of 2 with heavy metal levels of 47 mg/L of Chromium (Cr), 54 mg/L of Nickel (Ni) & 15 mg/L of Zinc (Zn) and Fluoride levels of 200 mg/L with acceptable levels being 20 mg/L, 10 mg/L, 10 mg/L and 30 mg/L respectively.
PROPOSED SOLUTION – PROCESS OVERVIEW & COMPONENTS
Wastewater to be treated fills the conical mixing tank, once this tank reaches the required levels; the wastewater is first treated to adjust the pH via dosing it with Magnesium Hydroxide Slurry. This slurry also has the added benefit of precipitating out some of the heavy metals. During pH correction, the wastewater is re-circulated through a re-circulation line that has an in-line pH probe to determine the Ph of the wastewater & maintain it at acceptable levels.
Once pH has reached acceptable levels, the wastewater is then pumped through a SS Bag filter (to remove the precipitated heavy metals) and then through a 24” Hydroxyapatite (HAP) filter (to remove any remaining heavy metals & Fluoride). The filtered wastewater is then pumped to waste via a flowmeter (flow to waste is controlled through the use of two diaphragm valves and a flow control line that redirects some of the filtered wastewater back to the inlet of the pump (to control waste discharge to drain).
The key steps & components that needed to be in place to address this particular problem are as follows:
- pH correction using Magnesium Hydroxide Slurry (Brucite) to stabilise pH to acceptable levels (> 7)
- The pH levels are controlled via a pH probe w/ Controller (controller is used as a pH monitor in this setup)
- The Brucite has the added benefit of also precipitating out the heavy metals
- Filtering out the precipitated heavy metals via a SS Bag Filter (10 µm)
- Final Polishing via a 24” HAP filter to remove any remaining heavy metals & Fluoride
KEY PROCESS STEPS
1. MAGNESIUM HYDROXIDE SLURRY (BRUCITE)
Due to the significantly low pH of the wastewater, a chemical addition needs to be performed to stabilise the pH. The pickling plant was using caustic solution to increase pH levels prior to PWT implementing its treatment setup. The caustic was replaced with Magnesium Hydroxide Slurry (Brucite) as it is not only capable of increasing pH but also capable of precipitating out some of the heavy metals in the solution. The increase in pH is also gradual with the Happy Fortune increasing pH plateauing around 8-9 with further addition of Brucite which is not the case with Caustic that tends to keep on increasing the pH as caustic levels increase.
The gradual increase in pH levels by Brucite also meant it was easier to integrate with the existing dosing setup that comprised of a 2” dosing line with dosing pump (this dosing setup is not ideal but as per the client’s request to keep the existing setup, this was the best solution possible). This means that even with a bit of over-dosing (which is likely with a 2” line), there will be minimal issues with exaggeratedly high levels of pH.
2. pH PROBE w/ CONTROLLER
A pH probe was put inline into the recirculation loop to monitor pH correction with Brucite slurry. The pH probe is connected to a controller box that is used to show the pH readings to the operator. For the initial setup, the client was looking at only using the controller box as a monitor to show the pH readings so that the operator can know when to Start/Stop during manual dosing. The client later engaged PWT to look at upgrading their existing dosing setup where the existing controller box will be connected to the new dosing pump to automatically control pH & dosing. A new dosing pump was put in place and connected to the pH controller box to fully automate the dosing process.
3. SS BAG FILTER
A Stainless-Steel Bag Filter w/ 10 micron screen was put in place to remove the precipitated out heavy metals, courtesy of the Brucite slurry, out of the wastewater. The bag filter was situated in a place next to an elevated platform for ease of use especially during removal of the clogged bags during service/maintenance.
4. HYDROXYAPATITE FILTER (HAP)
A 24” top to bottom flow Hydroxyapatite (HAP) Filter was put in place to perform final polishing on the wastewater to remove any remaining heavy metals & fluoride and bring them down to acceptable levels.
A top to bottom flow filter vessel was used instead of a backwash filter as per the client’s request. This was done in order to keep changes to the existing setup to a minimum. The HAP filter works by adsorbing the heavy metals from the wastewater while fluoride replaces the OH– ion in HAP and is removed.
5. ROTAMETER & SAMPLING POINT
A rotameter & sampling point were put in place downstream of the HAP filter in order to control the discharge of waste as well as to quality check the heavy metal & fluoride content in the treated water.
The discharge flow is controlled via a diaphragm valve and a recycle loop (recycle line put in place from outlet of HAP filter to inlet of pump feeding the conical storage tank) to redirect some of the treated water back to the conical tank in the case where discharge rate is higher than expected/required.
The sampling point is a ½” drain valve put in place just before the rotameter i.e., before the wastewater gets discharged. This sampling point gives operators/the client the ability to take samples of the filtered water and conduct their own testing to satisfy their quality checks as well as the government guidelines.
FOR ANY ENQUIRIES REGARDING HEAVY METALS REMOVAL OR ANY OTHER FILTRATION PROCESS, PLEASE FEEL FREE TO CONTACT US BY