Measuring Organic Compounds in Wastewater – Which Method Should I Choose? A Case Study from a

European Sustainable Sugar Manufacturer

This article compares three techniques which measure the organic content in wastewater (BOD, COD & TOC), and which is the most suitable analysis depending on the processing needs. Optimizing the method can lead

to quick ROI in many instances. Read on...By Alyson Lanciki & Peter Gulden

Suiker Unie is a company headquartered in the Netherlands which produces sugar, syrups, and other products originating from sugar beets. It is the European market

leader with its sustainable beet cultivation and processing methods, having won numerous awards. During each campaign of approximately 130 days, Suiker Unie processes on 2 sites in total six million tons of sugar beets, creating one million tons of sugar, and cleans 5,000,000 m3 of water before releasing it back into the environment. In 2009, a biogas installation was added to the Dinteloord site to recover organic compounds in wastewater, convert them into methane, and return the upgraded biogas to the Dutch energy grid. Sugar production regulations will be lifted in 2017 and plans are to increase output by about 10%.

Introduction

Sugar is an important foodstuff, consumed by everyone on the planet. According to Peter Gulden, the Production Leader at Suiker Unie for Biomass Fermentation, the average Dutch citizen consumes around 40 kg of sugar per year, and this number is even higher in other countries. The production of sugar and other agricultural products requires extremely large volumes of water. From farming and irrigation practices to washing the resulting produce, extracting compounds, evaporating and distilling, and even using the heat and energy from steam as utility, water is ubiquitous in all processes. The water needed in the sugar factory is mainly extracted from the beet during evaporation, and it is condensed and reused for washing.

Producing sugar from beets requires thorough washing of the vegetable slices, where a percentage of sugar is lost to the

rinse water. Suiker Unie Dinteloord prepares 27,000 tons of beets each day, equaling 40 tons of sugar lost in the rinse water. Before the water can be discharged to the environment, it must be analyzed and efficiently treated. Suiker Unie processes and cleans almost 2,500,000 m3 of water at the Dinteloord site each year, and utilizes an effluent treatment program combining both aerobic and anaerobic water treatment processes, releasing less than 3% of the total organic load upstream in the process. While aerobic treatments (holding ponds) are reserved for less contaminated water from their steam circuit, the biomass digester (Figure 1) and methane reactors reactors (anaerobic, Figure 2) are fed with influent containing a higher load of sugars. These reactors generate significant revenue for the company as the gas is cleaned and sold back to the power grid.

An overload of organic matter in the digester causes significant problems and downtime: bacterial death from pH changes, reactor shutdown, cleanup, and recommissioning procedures, which can be extremely costly. Until 2015, Chemical Oxygen Demand (COD) analysis was performed daily on site by the laboratory at Suiker Unie in order to measure the organic load of the influent and control the flowrate to the reactors. This measurement was based on 24 hour composite sampling—small amounts of influent collected every 15 minutes over the period of a day, combined in a 10 L tank for COD analysis. The company is allowed by law to discharge up to 125 mg/L COD back into the environment, therefore it is imperative that the wastewater treatment procedures are working efficiently. The maximum degradation which can occur (per reactor) is calculated around

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30 tons of COD per day, or 1400 kg per hour, equating to 3800 mg/L COD at high flowrates (800 m3/hour). During peak organic loads, the enriched influent can be diluted with less polluted water from the holding ponds to overcome overloading the methane reactors. However, the time-consuming COD measurement taken at such a low frequency does not ensure a constant organic load to the reactors. Peak loads are averaged in the daily COD results if they occur hours after the previous measurement was taken.

Other methods are available to measure the organic content in water, among them is Total Organic Carbon (TOC) a continuous, online, sum-parameter analysis. In late 2014 a Metrohm Process Analytics 7010 TOC Analyzer was installed at the Suiker Unie Dinteloord site at the inlet to the methane reactors to provide quick feedback about the stream (Figure 3). Over a three month testing period, a correlation factor was calculated and confirmed between the COD and TOC measurements. Suiker Unie now uses the 7010 TOC Analyzer to protect their investment: monitoring the organic concentration in their wastewater continuously in real-time (Figure 4).

Methods for Wastewater Monitoring

Wastewater originating from sugar production is especially problematic as it is filled with soluble, nutritional organic material which can lead to a severe depletion of dissolved oxygen levels due to microbial metabolism. In order to protect aquatic life, the organic content must be measured and treatments performed on very polluted effluents before this water can be reintroduced to the environment. There are many ways to measure the organic load, and therefore oxygen demand, in wastewater: Biological Oxygen Demand (BOD), Chemical Oxygen Demand (COD), and Total Organic Carbon (TOC). The duration of both BOD and COD analyses is quite long, which leads to missed/averaged

peak organic loads, or requires multiple analyzers at increased cost. The measurement of TOC occurs in minutes, giving improved time resolution for dynamic processes and does not involve the use of toxic chemicals as with COD analyses. TOC measurements can be used for real-time control in a process and also to continuously monitor effluent discharge to adhere to environmental regulations.

Issues with Organics in Biogas (Green Gas) Production

Other than being a major sugar producer, Suiker Unie is the biggest green gas producer in the Netherlands. In 2015, 22,000,000 m3 green gas was created between their two main production sites and sold back to the Dutch power grid. During a campaign, Suiker Unie can generate 1.5 million euros worth of gas, thus it is very lucrative and in the best interest to keep the reactors running at maximum efficiency. Over the first six years of service for their biogas reactors, the organic content of the wash effluent was monitored by COD analysis in the laboratory. This analysis took place once every 24 hours from a 10 L composite sample (created by aliquots taken from the effluent stream every 15 minutes) to ensure the proper level of nutrients reached the bacteria. However this measurement frequency missed some significant overloading periods, which lowered the pH of the reactor, killing the activated sludge. A shutdown of the reactors and remediation procedures followed, costing more than €200,000.00 after which Peter Gulden, Biomass Fermentation Production Leader, began looking for other methods to better control this process.

Quantifying Organic Content: BOD, COD, and TOC

Measuring the organic levels in wastewater has different implications for each industry, depending on the reasons for performing the analysis. The three main analyses used are BOD, COD, and TOC and they vary widely, which can be confusing when determining which method is best for your process.

• BOD: The BOD is defined as the amount of oxygenconsumed by microorganisms in 1 L of water within fivedays (BOD5) in order to reduce the organic load. Thismeasurement is performed within a closed system to ensureno external influences will affect the oxygen concentrationsinside of the bottle. If calculating the Ultimate BOD, thismeasurement can take longer than five days. This analysis has

Figure 1. The biomass digester installed in 2009 by Suiker Unie

Dinteloord to create biogas from their wastewater.

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very specific pH, nutrient, and chemical requirements, and is completely unsuitable for process control. Results are based on the activity of microorganisms and can be biased if there are any toxic substances also present in the wastewater.

• COD: The COD is a measurement of the oxygen requiredto oxidize all the organic matter present in a sample. This isperformed titrimetrically such as in the norms DIN 38409,EPA 410.1, and ISO 60660. It differs from BOD in that it ismuch faster (between two and four hours per analysis) andrequires a strong oxidizing agent for the task, rather thanmicrobial processes.

Potassium dichromate (K2Cr2O7) is used to fully oxidize allorganic matter under extremely acidic conditions to createCO2, the final product. An excess of the dichromate mustbe present in the sample, and after the oxidation processis complete, this excess reagent needs to be quantified viatitration. After adding the oxidizing reaction solution andsilver sulfate (Ag2SO4) catalyst, the mixture is heated for atleast 2 hours before cooling and beginning the titration. The

titrant ferrous(II) ammonium sulfate is added, and the Cr6+ in solution from the K2Cr2O7 is reduced to Cr3+, oxidizing Fe2+ to Fe3+– the endpoint noted potentiometrically.

In this method, especially toxic reagents are used. In samples which contain large amounts of chloride, complexing with mercuric sulfate (HgSO4) is recommended in order to avoid interferences from the additional oxidation of Cl-

and formation of Cl2 gas. The handling and disposal of the reagents is important and highly regulated, since they are extremely poisonous. In more and more countries, the use of mercury is forbidden.

Although some industries still use COD as a measure of the organic content of their process waters, many are moving away to monitor and control their water treatment processes with online TOC process analyzers.

• TOC: The measurement of TOC has entered the market as afaster and alternative method compared to BOD and COD.TOC is a quick online method which oxidizes the entireorganic content of the sample, measured as a sum-parameter(as CO2), meaning it gives no specific information about theindividual organic compounds. A liquid sample is taken andthe inorganic carbon is removed by acidification and purgingof the sample. The organic carbon components are oxidizedto CO2 which is then directed to a non-dispersive infrareddetector (NDIR), where the CO2 is detected at a specificwavelength.

TOC Methods

Digestion of the organic components can be done in various ways. One method is the catalyst-assisted high temperature digestion, which burns all organic material in the sample between 650°C and 1200°C. Determinations are carried out batch-wise since the combustion tube in the furnace has to be refilled with an injection needle with each analysis. However, power consumption

Figure 2. Methane reactors on site at the Dinteloord Suiker Unie facility.

Method Suitability

For BOD and COD analyses, the oxygen content needed to stabilize organic matter is the foundation of the measurement. However, the reported values may differ based on the oxidation states of compounds within the sample, whereas the carbon concentration (measured by TOC) remains constant. Therefore, it is clear that TOC is the most suitable method to determine organic content, as the result is independent of different oxidation states in the sample. The TOC value provides a quick, easy, and accurate way of assessing the amount of organic substances in a sample stream without the need for toxic chemicals, unlike COD analysis.

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Figure 3. The Metrohm Process Analytics online 7010 TOC Analyzer in a protective shelter analyzing the influent to the biomass digester at Suiker Unie Dinteloord. This analyzer has replaced the old method of COD measurement at the facility and helps control

the amount of organics fed to the bacteria inside of the digester.

to keep the furnace running for constant TOC analysis may be quite costly.

An effective and economical oxidation method is the wet-chemical digestion with UV light using a strong oxidizing agent, usually persulfate (S2O8

2-). The analysis takes place continuously, and offers both high sensitivity and low maintenance for the user. The UV-persulfate digestion is the basis for the Metrohm Process Analytics online 7010 TOC Analyzer, and with no need for external air utility, consumables cost less than €600.00 per year. The system with its simple device configuration and low maintenance requirements has been developed based on years of experience with TOC analyzers for online operation.

Determining the Correlation Factor

Correlating the results between COD and TOC can be performed with long-term sampling and statistical analysis using the results from both techniques, although this ratio must be established at each sampling location. The COD-TOC correlation is based on

the oxidation state of the compounds in relation to the amount of carbon present. This ratio may differ for the same process stream if different sampling locations are used, as well as for a large variability in the organic species present.

Suiker Unie’s two methane reactors can treat up to 60 tons of sugar daily to create approximately 22,000 m3 of green gas,

Figure 4. TOC vs. Biogas generation over the course of 24 hour period.

Pink: TOC concentration Red: Biogas generation

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and any downtime can have significant consequences not only for the gas production, but also for the wastewater, which must contain less than 125 mg/L COD before it can be returned to the environment. For the correlation study, the 24 hour COD measurement was compared to a “composite” 24 hour TOC measurement (averaged value from multiple measurements on samples taken every 15 minutes, exactly how the daily COD sample is created) each day over a period of three months (Figure 5). A factor of 2.67 was determined between the COD and TOC measurements, meaning that Suiker Unie can still calibrate with the same standard sugar solution as before. Now they have no need for toxic chemicals containing mercury and silver, and have complete, real-time control over the organic load influent to the methane reactors.

Conclusion

It is nearly impossible to identify and quantify the myriad organic compounds present in wastewaters, which makes the sum-parameter measurement of Total Organic Carbon more than simply a shortcut to determining the total amount of organic substances in a sample. TOC measurement is not only convenient, but the only way of arriving at a conclusion about organic contamination and carbon content in such

samples. There are no toxic chemicals nor issues with different oxidizable species in the sample, making TOC a straightforward measurement. Monitoring and controlling dynamic processes is made easier with this fast, online measurement. By spending around €25,000.00 for a continuous online Metrohm Process Analytics 7010 TOC Analyzer and an analyzer shelter, and €200.00 on operational costs over a 130-day campaign, Suiker Unie has protected its significantinvestment in sustainability, saving €100,000.00 per reactoron activated sludge plus the millions of euros generated byselling green gas back to the Dutch power grid. For moreinformation, please visit www.metrohm.com.

Other Applications of Continuous Online TOC:

• Effluent Consent Monitoring and Control

• Monitoring Breakthrough in Boiler Water

• Monitoring Surface Water Discharges

• Monitoring and Control of Coagulation

• Monitoring and Control of Recycled Water

• Organic Contamination/Build Up in Processes

Figure 5. A 3-month correlation study between the COD results and equivalent TOC measurements at Suiker Unie Dinteloord.

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Alyson Lanciki has a Ph.D. in Analytical/Environmental Chemistry from South Dakota State University, and a B.S. in Environmental Chemistry from the State

University of New York College of Environmental Science and Forestry. She has worked at Metrohm Applikon B.V. since 2013 as an Applications Specialist and is currently the Technical Writer for Process Analytics.

Peter Gulden, Production Leader at Suiker Unie for Biomass Fermentation. Peter graduated from higher laboratory education and began working at the Suiker

Unie research laboratory in 1982. In 2008, Peter became head of the production laboratory in Dinteloord, the Netherlands and was also responsible for the water treatment and methane reactors. Since February 2015, Peter is now the Production Leader of the biomass digester and methane reactors.

About the AuthorsQUOTES (Peter Gulden):

Preliminary testing of the 7010 TOC Process Analyzer:“The guys in the control room saw this [TOC] figure the whole campaign, but I wanted to see if it was good enough to use it because if it’s broken down every 2 weeks, there’s a moment where they don’t look at it.”

After testing the 7010 TOC Process Analyzer:“It’s a simple system, nothing can happen.”“The calibration was only with a sugar solution.”

After full integration of the 7010 TOC Process Analyzer:“For me it was very satisfying. One time we saw there was a problem in the factory, there was 200 tons of sugar put in the water circuit and immediately it came to the methane reactors, and we saw that immediately, [with an] alarm”

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