CASE STUDY
Water Technologies
& Solutions
Purification & Disinfection Systems
Ozone Solutions for Micropollutants Treatment at all Positions in the Water Cycle: Drinking Water to Wastewater
Laurent De Franceschi*, Christopher Huynh*, Dr. Michael Petri**, Max Schachtler***
*SUEZ Water Technologies & Solutions, ** Zweckverband Bodensee-Wasserversorgung, ***ARA Neugut
Background
Concentrations of trace contaminants in all positions within the global water cycle continue to increase as the proliferation of pharmaceuticals, cosmetics, and personal care products increases. The adverse 'cocktail' effect, result of the combination of a wide variety of these contaminants of emerging concern (CEC) or micropollutants, onto the environment and human health have been now proven through many studies.
Consequently, anthropogenic micropollutants are a significant concern today and are quite ubiquitously distributed in water bodies. Concentrations of these compounds is a way to differentiate them through the global water cycle.
In urban and industrial wastewater contains, in general, the highest concentration are found and this must be a key targeted location to remove them. In addition, diffused sources including ground water and surface water, often the origin of drinking water production, must be considered for treatment to address these contaminants.
Eco-toxicity, bio-accumulation and bio-amplification, as well as endocrine disruptors, antibiotic resistance or cocktail effects are terms usually used to qualify harmful effects on environments and human health of these anthropogenic compounds.
We are at the beginning of the story. The question now is not whether or not to treat these compounds, but rather how best to treat for them.
Ozone treatment solutions in the water cycle – return of experience
Among treatment solutions available and applicable in the global water cycle, ozone is a technology proven to be efficient and cost effective for removal of micropollutants. Regarding treatment in wastewater, Switzerland took the subject to heart in the early 2010s.
Through public research organisations, two main solutions were qualified: ozone and activated carbon. Several criteria enable the assessment of which technology should be applied. In this sense, a combination of both technologies is a potential solution.
a. Micropollutants in Water
As a concentrated source of various micropollutants, wastewater from industrial and urban activity is an appropriate site to target them before discharge. In ARA Neugut WWTP (Switzerland) the first micropollutant treatment was implemented in 2014. Several relevant designs and solutions were provided from ARA Neugut (M. Schachtler) for treatment optimization of this first full-scale plant:
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BEAR is the control system developed to optimize ozone dosage. Based on delta UV254nm measurement in between inlet and outlet ozonation stage, in combination with water flowrate and a smart algorithm, the control solution aims to optimize ozone dosage in a way that treatment efficiency is 82%, thus achieved elimination without overconsumption of ozone. In Switzerland, regulation ask for 80% removal rate of average 12 substances considered as indicators.
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Ozone dosage optimization conducted onsite resulted in a significant decrease of specific dosage from 0,5 – 0,8 g O3 abs. / g DOC in design phase to 0,4 +/- 10% g O3 abs. / g DOC in current operation.
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The elimination of the 12 lead (indicator) substances at ARA Neugut, from year 2016 to 2018, is detailed in the subsequent graph:
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Injection of ozone in multiple points is a method also promoted by SUEZ for this process optimization. At Neugut, this was implemented by splitting the dosage and adding half in the first and half in the third chamber of the ozonation reactor.
All in all, a significant operating cost reduction has been conducted with saving in energy and Liquid Oxygen of more than 30% compared to the original design estimates.
b. Compounds of Emerging Concern (CECs) in Water Reuse
One step forward in the global water cycle is the production of potable water from wastewater, or potable reuse. Reuse is a necessity in areas where water scarcity is present. Indirect Potable Reuse (IPR) involves an intermediate water body such as a lake while Direct Potable Reuse (DPR) is when the WWTP is directly connected to the drinking water plant. In both IPR and DPR, Ozone + Biofiltration is a proven treatment option which, in addition to disinfection and pathogen destruction, enables efficient removal of CECs.
The study carried out in Gwinnett County IPR, aimed to evaluate Ozone + Biofiltration for DPR as an alternative to Reverse Osmosis (RO) based treatment. Piloting done showed that with a blend containing 15% of wastewater from F. Wayne Hill WRP met all Maximum Contaminants Levels (MCLs) required by regulation.
Ozone dose applied at F. Wayne Hill WRP was respectively 3 mg/l at pre-ozonation and 1,5 mg/l at post ozonation.
c. Emerging Contaminants in Drinking Water
By moving to the next step in the global water cycle, use of ozone for drinking water treatment began near the turn of the 20th century (Bon voyage – Nice – 1907) and currently ozone is considered a best practice for this application.
Main goal claimed is generally disinfection, but due to high oxidative properties of ozone, reactions with several mineral and organic compounds are unavoidable and furthermore welcomed. Lake Constance Water Supply in Sipplingen, close to Lake Constance in Germany, implemented ozone early in the 1970s for disinfection purpose.
Ozone was chosen for its efficiency and flexibility vs chlorine. As a secondary goal, due to potential risk of organic pollution in the water source, Lake Constance, ozone is considered as a barrier to prevent presence of emerging contaminants in final potable water produced by the plant.
Over the years, the presence of several organic contaminants from various categories (pesticides, pharmaceuticals, hormones, anticorrosive, detergents…) have been identified in water from Lake Constance. As a whole, these contaminants are removed to concentration lower than LOD (limit of detection). Following figure highlights the removal of pharmaceuticals compounds from raw water:
Removal rate of ozone treatment, in combination with the subsequent filtration stage, is very high and > 90% for many compounds. Control of ozone dosage at Sipplingen is done through a simple dissolved ozone set point at 0,75 mg/l in the inflow of the intermediate reservoir. Use of ozone, in combination of biological activity of following rapid sand filtration plays the role of barrier against compounds of emerging concerns presence in potable water. Key considerations for design and implementation As the relevant treatment step in the global water cycle, ozone for removal of micropollutants in treated water from a WWTP should take into account key considerations. Some, among them are resulting from Swiss experience:
With the implementation of an online UV 254 nm control system, ozone dosage can be optimized. In ARA Neugut optimization went to 0,4 g ozone abs. / g DOC, compared to first figures used for plant sizing for a removal rate of constant 82%. Sizing range of ozone dosage for such a removal rate is nowadays usually decreased at design stage, from 0,4 to 0,8 g ozone abs. / g DOC.
Splitting ozone dosage in at least two injection point in the contacting system allows significant reduction of OPEX costs and aids in successful mitigation or no significant formation of bromate.
Point 1 & 2 aims both to reduce as much as possible dissolved ozone presence within the water. This, not only for OPEX optimization but also in order to mitigate bromates formation, which occurs with high concentration of bromide within the water matrix. Concentrations higher than 100 - 200 µg/l requires careful attention and ozone technology may not be suitable in all cases.
Combination with activated carbon is a possible solution which aims to take the best from both technologies.
In the most effective cases, ozone is followed by a biological stage, either actively or passively generated. Usually it is sand-filtration (biofiltration) whose main “raison d’être” is degradation by the microbial species of intermediate and labile compounds formed during ozonation.
From an economical point of view
Cost of micropollutant treatment is sensitive because it comes in addition to the existing OPEX of a WWTP. In Switzerland, cost assessments regarding this quaternary treatment were provided in several studies from public organizations.
