Warm Weather Water Treatment: Optimizing Coagulation With Standby Advanced Oxidation

By Emma Flanagan

Seasonal variations and peak events can impact the water quality in ways difficult to anticipate, requiring tools and monitoring methods to optimize drinking water treatment at a moment’s notice.

Seasonal occurrence of algae blooms in surface waters brings about taste and odor (T&O) problems, the risk of disinfection byproducts (DBPs), and disturbance to water treatment systems. The coagulation stage is recommended for their removal and to regulate occasional raw water quality fluctuations to ensure minimal impact on subsequent processes. In practice, owing to the variable condition of the water and low degradability of certain compounds, achieving the desired results with a single conventional method is difficult. Advanced oxidation processes (AOPs) are attractive alternatives for traditional water treatments and are receiving considerable attention. The proposition to optimize coagulation with AOPs is promising for organic pollutants degradation because, as a preoxidant, they can break down organic and inorganic contaminants to total decomposition.

Quantification Of Organic Matter

When we get access to data that can accurately pinpoint a problem we are able to determine what changes will make the most impact on improving water quality without wasting resources. Sensors that monitor organic matter and turbidity are widely used to determine the required dose of coagulant and flocculant. Turbidity only accounts for half the story; other quality parameters such as pH, suspended solids, conductivity, temperature, and total organic carbon (TOC) are also good indicators, but they are not so effective when natural organic matter (NOM) is the culprit of the turbidity problem. Adding unnecessary coagulant causes unwanted sludge removal and higher chemical cost. NOM is a broad term employed to describe heterogeneous mixtures of organic compounds derived from both plant and animal sources. NOM is omnipresent in natural waters but is not all-natural as the name suggests; some are man-made, such as the ones contributed by agricultural activities. Prior to the 1970s, the negative health effects of NOM in water supplies were unknown, and its presence was primarily viewed as an aesthetic problem. With the discovery of DBPs in the mid-1970s, caused by incomplete oxidation by halogenated chemicals — mainly chlorine-based — with certain organics, their removal became a priority for regulators and utilities. There were only two approaches for consideration: reduce chlorination or replace it with other water disinfectants, or enhance NOM removal to reduce DBP precursors.

TOC is an important assessment of the organic pollution and measures the total amount of organic carbon present in water. When dealing with a high concentration of DBP precursors and T&O issues, other quality parameters are recommended to provide different characterizations of the organic content: dissolved organic carbon (DOC), biochemical oxygen demand (BOD), and chemical oxygen demand (COD).

DOC is a measurement of the fraction of TOC in water that can pass through a filter sized from 0.22 to 0.7 micrometers (μm). It is relevant because a high proportion of DOC is biodegradable and supports growth of microorganisms as a food supplement and source of energy and carbon. It will facilitate biological regrowth in filters, membranes, and distribution systems.

BOD measures the amount of dissolved oxygen (DO) that is used by aerobic microorganisms to break down organic matter in water at a given temperature and over a specific period of time. It is commonly expressed in mg/L. BOD values are generally determined and evaluated in association with other parameters, and this makes them more useful in formulating predictions.

COD can easily quantify the organics in water. It is an indicative measure of the amount of oxygen that can be consumed by chemical reactions in a measured solution. Just as with BOD, it is expressed in mg/L. It is an alternate method to BOD in estimating oxygen consumption by bacterial activity when breaking down organics, with the advantage of only taking a couple of hours to complete. It is used in conjunction with the BOD test, as BOD/COD ratio, to estimate the amount of nonbiodegradable organic material, more often in wastewater, and more recently also popularized in natural waters due to the influence of man-made contamination. BOD/COD ratio can be used as a crucial attribute for characterization of pollution measurement. One practical example is a rapid drop in the value as an indicator of an industrial spill or illegal discharge of untreated wastewater effluent upstream from the water treatment facility. But when put into practice, waiting five days or even five hours to interpret a rapid drop in the BOD/COD value renders this tool obsolete. It will only work with real-time data.

UV-Visible Spectrophotometry

The analyses of TOC, DOC, BOD, and COD are not simple, require chemical reagents, and are time-consuming and expensive. Real-time monitoring of these organic parameters is often not feasible, and they don’t support optimization of a treatment process to adjust for short-notice, seasonal, and short-term variations in the composition of the raw water source. Fortunately, there is a newer available test called ultraviolet-visible spectrophotometry (UV-VIS) that uses spectral absorption, a measurement of the amount of ultraviolet and/or visible light absorbed by a water sample of a given path length. It is a technically simple and fast method requiring neither expensive measurement equipment nor chemical reagents. This has resulted in the increasing use of a method that measures the absorbance of light by organic compounds present in waters at 254-nanometer (nm) wavelength, the UV254 test, as the most practical organic water-quality parameter. Results are more specific to aromatic compounds correlated to organic carbon, color, and the reactive NOM that contributes the most to form DBPs.

The main ecological impact of organic pollution is the oxygen depletion in water; therefore, oxygen demand-based methods are very important. The overall quality of water depends on the concentration of the aggregate organic constituents. Since the absorbance of matter in water is directly proportional to the concentration, according to Beer’s Law, a correlation can be made to any aggregate organic parameter. UV-VIS spectrophotometers can associate UV254 absorbance with other wavelengths and measurements. The specific UV absorbance water analysis data can be collected in combination with laboratory data to develop a site-specific correlation for the parameter of interest (customized calibrations can combine the most influential data for the location and have much higher accuracy). This would allow the instrument to continuously convert the real-time measured absorbance data into a representative mg/L concentration of each parameter and provide an accurate value of BOD, COD, DOC, and TOC.

Another important calculation that assists with determining the potential of DBP formation is specific ultraviolet absorbance (SUVA). It is a characterization that allows the aromatic organics that dominate the UV254 measurement to be normalized for DOC concentration in the water. It can be obtained by dividing the UV254 absorbance by the DOC of a water sample. This ratio can describe the hydrophilicity and hydrophobicity of NOM. SUVA values of 4 to 5 L/mg-m¹ suggest that NOM is dominated by more hydrophobic, aromatic, higher-molecular-weight organics and DOC controls coagulant demand. It indicates there is a greater potential for the formation of DBPs. In contrast, SUVA values less than 3 L/mg-m are associated with more hydrophilic or nonabsorbable DOC and there will be minimal removal by coagulation.

Oxidation-Reduction Potential

How can we maximize operation with this tool?

UV254 testing provides a real-time surrogate measurement of NOM that enables a competent and continuous control of the water treatment processes during seasonal and sudden event variations. Pairing real-time water quality data with an effective real-time automatic controller dosification system will secure that at the preoxidation stage the system will apply the right amount of chemicals when needed to normalize the water parameters precoagulation. The monitoring of the oxidation-reduction potential (ORP) method is inexpensive, flexible, and effective at determining the amount of oxidant to add to the water to get the desired residual, expressed as oxidative energy in millivolts (mV). ORP does not directly measure the concentration of an oxidant and it correlates weakly with chlorine residual. ORP values are better indicative of dissolved oxygen content, microbial activity, and chemical processes. A minimum ORP of +300 mV is required to oxidize organic compounds generated from algae that contribute to T&O issues, while recalcitrant contaminants and pesticides will need an ORP above +400 mV. Water with Escherichia coli contamination needs to go a bit higher and maintain the ORP above +500 mV.

A system that combines real-time water quality data and ORP automatic control can avoid the most critical challenges faced by water utilities when inadequate removal causes T&O problems, faulty biofouling control, and DBP violations in the facility and within the distribution system. A complete UV254/ORP solution can operate 24/7 to minimize challenges with direct impact on operational savings through optimizing coagulant and dosage, good floc formation and settling, long filter runtimes, less backwashes, less sludge production, less membrane fouling, good hydraulic capacity, and effective ion exchange/adsorption processes.

Precoagulation Oxidation

Several oxidants are available to treat raw sources and maximize coagulation. Aluminum (alum) and ferric salts are well suited for treating negatively charged NOM and turbidity, and are therefore the most widely used coagulants in the U.S.

A recent study by the Tokyo Institute of Technology investigated the effect of prechlorination contact time on the control of cyanobacteria Microcystis aeruginosa by coagulation. During this analysis, standard prechlorination at high chlorine doses did not show significant improvement in the removal by coagulation. Increasing contact time to 30 minutes showed better algae removal with adjustment of the alum dose. However, at lower chlorine doses, DOC and UV254 tended to increase when the contact time increased. DOC increase indicated the release of intracellular organic matter (IOM) from M. aeruginosa, consistent with the fact that chlorine has been found to damage the algae cell wall and release IOM. The increase of DOC concentration correlated with higher UV254 values and the increase of dissolved organic matter (DOM), which inhibited the aggregation of algae cells and increased the alum dose.

This study concluded that the original alum dose that was sufficient for removing the algae after high chlorine dose was not sufficient to remove organic compounds produced after prechlorination, and the bacteria could not be removed by coagulation unless more alum was added. Moreover, due to the increase of DBP precursors found in this study when a high chlorine dose is used, the prechlorination contact time should be less than 30 minutes.

Another research work by Sun Yat-Sen University in China explored enhancement of chlorination with UV radiation and the resulting advanced oxidation process (UV/Cl AOP) of combining hydroxyl radical and chlorine species, HCL- and OCl-, in the degradation of four often-detected T&O compounds: geosmin (GSM), 2-methylisoborneol (MIB), benzothiazole (BT), and 2-isobutyl-3-methoxypyrazine (IBMP). The UV/Cl process effectively degraded the four compounds. Increase in chlorine concentration caused the reactive chlorine species (RCS) to be more dominant of the process and more specific to target compounds. RCS could not degrade GSM. Excess RCS were also scavenging the hydroxyl radicals and the process was more sensitive to pH variations. Kinetic analyses of the competitive reactions showed that in the UV/chlorine process the hydroxyl radical primarily contributed to the degradation of the T&O compounds. The effectiveness of the experiments was directly related by their capacity of producing hydroxyl radicals.

Use of chlorine dioxide (ClO2) has grown significantly in drinking water treatment as regulations addressing DBP formation have become increasingly stringent, and it can be used in the preoxidation stage to improve coagulation. Because of its selective reactivity, in comparison with chlorine, ClO2 is an effective control strategy for taste, odor, color, iron, and manganese removal, as well as waterborne pathogen control, while minimizing halogenated DBPs. It oxidizes floating particles and aids the coagulation process and the removal of turbidity from water. For the preoxidation and reduction of organic substances, the water quality determines the contact time that will be needed, typically between 15 and 30 minutes. ClO2 is relatively expensive, presents a high level of danger, and induces the formation of inorganic chlorites and chlorates.

Preoxidation with ozone (O3) has been reported to be more effective than chlorination and chlorine dioxide in drinking water treatment processes. Bench studies consistently demonstrated reduction in filter effluent turbidity, extended runtimes, increased application rates, and better biofouling control. Ozone has been credited with the destabilization of particles by several hypothesized mechanisms such as assisting with the adsorption of organics to metal oxides and creating organic polymers that are easily coagulated. Large numbers of water treatment plants (WTPs) report to have improved particle removal after ozonation as well as lower dosages of coagulant, many in the range of 20 percent to 50 percent. These benefits are normally not predicted by lab-scale studies, and models do not exist to predict that ozone water treatment will improve coagulation and flocculation. The effect is normally observed upon full-scale operation. By the same token, practice has shown the results are not consistent and that preozonation can improve coagulation for some waters but not for others. Also, even for waters amenable to this process, seasonal variations can render preozonation temporarily ineffective. In certain conditions, increasing ozone concentration may be beneficial for turbidity but detrimental for UV254 removal and inhibit flocculation. Ozonation increases biodegradability of large NOM molecules but does not oxidize them completely to carbon dioxide and water. It does not destroy most algae cells at the dosages below 3 mg/L, frequently used to aid coagulation and flocculation. One theory is that in peak algae season, for example, higher ozone concentration may result in a fragmentation of large algae molecules and increased release of smaller organic molecules, more likely to impair coagulation by increasing the coagulant demand or by adsorbing and stabilizing mineral particles. Algae type and growth phase would play a part in this scenario.

The lack of theoretical basis for the coagulating effect of ozone has hindered its application for this purpose. Another consideration is that while ozone can oxidize precursors of chlorinated DBPs, it is not exempt from creating potentially harmful substances due to reactions with bromide ions giving rise to bromate. On the other hand, ozone has a relative high cost and complex operation, and must be generated onsite.

When preozonation is desired for other treatment objectives, such as direct disinfection and oxidizing benefits, potential detrimental impacts to coagulation can be offset by adjusting coagulant type and dosages, pH adjustments, biological processes, and combining with or switching to a different preoxidant. Just as with chlorination, ozone performance can be elevated to that of advanced oxidation by augmenting hydroxyl radical production with UV light radiation. However, the presence of color and turbidity will impair the effectiveness of AOPs using UV, a high-cost proposition to begin with. Other studies have enhanced ozonation with hydrogen peroxide to achieve advanced oxidation. Hydrogen peroxide (H2O2) addition is much less costly and less complicated than UV, but costs associated with the chemical and its residual quenching agents, and the operational challenge of balancing peroxide quenching with the needs of secondary disinfection, are a major disadvantage.

When considering the different approaches, UV/Cl, UV/ O3, and O3/H2O2, a sharp decline in UV254 was observed immediately following AOP activation. Overall, with respect to particle destabilization and aggregation, filtered water turbidity, filter productivity, performance consistency amid water quality variations, and chemical coagulant cost savings, AOPs have been found to be more effective and dependable as a coagulant aid than the oxidants alone. The reality is that the capital investment, blueprint requirements, and the additional expenses and complexity of the operation of traditional AOPs are not easily afforded by water treatment facilities. When regular oxidation can manage the organic load for the majority of the time, investing in a costly advanced technology to help overcome recurring episodes and stay in compliance is an impossible sale.

Second-Generation AOPs

AOPs begin with a source of oxygen (ozone, hydrogen peroxide, or sodium hypochlorite) and an energy source — external, such as UV radiation or electricity, and internal, such as iron salts — which is used in combination with hydrogen peroxide and is known as the Fenton process. H2O2 and iron generate hydroxyl radicals through a catalytic process based on electron transfer between H2O2 and iron ions. The hydroxyl radical production is extremely fast and in numerous studies they have found it remarkably effective as a control method to support coagulation in removing NOM from organic-rich waters and peak events. Under optimal pH, H2O2 ratio, and ferrous ion (Fe2+) dose, it can achieve greater than 90 percent removal of DOC and UV254 absorbance, with the advantage that no energy input is necessary to activate the hydrogen peroxide. However, it is very expensive and needs skilled operation. The additional water pollution caused by adding the iron salt requires disposal, and the process demands continuous supply of feed chemicals and pH correction.

A new family of AOPs has emerged with diverse approaches to the fundamental principles of Fenton’s reagent and the objective to enhance the original process and minimize its disadvantages. Increasingly popular in the water industry is the mineral oxychloride solution, a new-generation AOP that optimized Fenton’s chemistry by integrating the metal catalyst in the form of multiple transition mineral chelates stabilized at high pH in a water-based solvent. In other words, the mineral oxychloride solution takes on the catalytic role the iron salt has in the traditional Fenton process, and the water to be treated will support production of hydrogen peroxide, autogenerated as an intermediate reaction.

When the mineral oxychloride is activated with water, the chemical reaction causes decomposition of H2O molecules and sparks a chain of redox processes, based on the effect of direct and indirect reaction mechanisms with compounds that contain double oxygen bonds. AOPs have three phases: initiation (the spark of unstable oxygen species), propagation (known as radical chain reaction with net increase of reactive oxygen metabolites), and termination (when reactive species combine to form stable compounds and no new species are produced). The catalyst fuels the entire development. In the case of the mineral oxychloride solution, mineral catalysts are chelated with oxygen precursors to maximize redox energy and sustain reactivity for a long time. The catalytic effect of the transition minerals perpetuates the regeneration of oxygen species within an oxygen complex, in an ample pH range and at various oxidation states: superoxide [O₂- ], hydroxyl ion [OH- ], hydroxyl radical [OH ], nascent oxygen [¹O₂ ], oxygen ion [O- ], hydroperoxyl [HO-₂ ], and peroxide [O₂²- ]. The synergy of the oxygen complex causes a significant increase of ORP compared to any of the reactive species alone.

(Left) Weir post-chlorination, before the ozone contactor at the beginning of the study. (Right) Close-up of same weir after system was maintained at +350 mV with advanced oxidation technology.

A Case Study: T&O, Bromate, Aging Ozone Generators

A full-scale pilot was conducted during the summer months at a water treatment plant in California, near the San Francisco Bay Area, with surface water from the Sacramento-San Joaquin Delta.

Operating parameters: 10 MGD, maintained 0.5 mg/L chlorine residual; primary coagulant-alum @ 35 mg/L; polyamine-based coagulant aid: 1.5 mg/L, which only produced light flocs that needed longer settling times; preoxidation with ozone maintaining a 0.2 mg/L residual.

Challenges: Raw water pH > 8.5 with high bromide and high NOM; considerable amount of T&O complaints during the summer; aging ozone system operating at max capacity needed upgrade.

Treatment adjustments: We had a multifaceted approach maximizing coagulation with better-performing chemicals in addition to replacing prechlorination: 5 percent acidified alum was used to reduce raw water pH; the coagulant aid was replaced with an ACH polymer blend to increase molecular weight; the mineral oxychloride was applied at 5 to 10 mg/L (added) with a target ORP of +350 mV, to replace chlorination (0.5 mg/L residual).

Results:

• Ozone generators reduced operation to 40 percent capacity (down from 95 percent) to maintain same 0.2 mg/L residual
• ORP going into the ozone contactor was +350 mV
• Biofilm formation and deposit formation in the weirs was reduced
• Heterotrophic plate counts dropped to 10 colony-forming units (CFU) from 273 CFU
• UV254 absorbance dropped 50 percent from 0.044 to 0.022 μm/cm
• Total trihalomethanes (TTHM) dropped 41 percent and haloacetic acids (HAA5) dropped to non-detect
• Turbidity dropped 50 percent from 2.2 to 1.1 nephelometric turbidity units (NTU)
• TOC dropped 73 percent from 4.4 mg/L to 1.5 mg/L
• Bromate formation dropped from 18 μg/L to non-detect
• No T&O complaints, while neighboring WTP continued to have T&O issues all summer
• Significant savings in chemical cost, energy consumption, and filter performance, with no violations

Conclusions

The mineral oxychloride solution is an effective driving force of ORP energy when applied to any water system as a stand-alone treatment, or in combination with chlorinated compounds. This is consequential to a very high yield production of hydroxyl radical, its main oxidative agent, well established as a nonselective reagent that will decompose most compounds to carbon dioxide and inorganic salts. It has a minimum shelf life of six months. This makes the mineral oxychloride solution an ideal product to have on hand to implement advanced oxidation in any size facility with an UV254/ORP combination, for all-year-round water quality control and optimization of the coagulation process.

The mineral oxychloride solution is sold with the trademark Biohydrox ™ by Envirocleen LLC (www.envirocleen.com).

Reference:

¹ SUVA is calculated by dividing the UV absorbance at 254 nm (cm-1) by the DOC, dissolved organic carbon, (mg/L) of a water sample, expressed in units of L/mg-m. (https://realtechwater.com/blog-post/specific-ultraviolet-absorbance-suva-and-uv254/)

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About The Author

Emma Flanagan is a water consulting professional specializing in applications of homogeneous catalytic generation of reactive oxygen species with transition minerals. She holds an MS degree in sanitary engineering from IHE Delft Institute for Water Education in the Netherlands, the largest international graduate water education facility in the world. Emma stays busy doing research, educating, and providing consulting services.