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Digester optimization helped by new thermal wet gas flow meters

The Kurz Wet Gas Flow meter allows multi-phase digestion to be effective and reliable.

By Ethan Webster

Wastewater treatment plants are under pressure from growing communities and more demanding regulations. For treatment plants in growing communities, the volume of wastewater that can be treated is in part determined by the capacity of the plant’s digesters. Typically, to increase capacity, treatment plants will build additional digesters. This requires large capital expenditures.

New developments in anaerobic digester controls allow a single digester to process a greater volume of solids in a shorter period of time by isolating and treating the phases of digestion separately. With multi-stage anaerobic digestion, the stages of treatment can be optimized for the specific conditions which bacteria prefer. Pre-treatment methods can be used prior to anaerobic digestion to improve digestion efficiency.

Among these, temperature phased anaerobic digestion (TPAD) is one promising method with a relatively low energy input and capital cost. TPAD occurs by separating the digestion process over time and precisely heating the different stages.

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The optimized anaerobic bacteria can convert organic matter at a far greater rate than those in a conventional digester.

There are three distinct steps during anaerobic digestion, each performed by a different group of microorganisms:

  1. Hydrolysis – proteins and fibres are broken down.
  2. Volatile acid fermentation – products of hydrolysis are converted into organic acids.
  3. Gas forming step – organic acids pro­duced during the acid phase change the pH and allow methane forming bacteria to proliferate.

Hydrolysis and methane formation can be enhanced by increasing operating temperature. However, acid formation is adversely affected by this.

Unanticipated changes in sludge quantity, or content, can create an imbalance of bacteria. The plant may receive industrial or agricultural waste with widely differing properties and the biochemical oxygen demand of different feed stocks can vary greatly, resulting in inconsistent gas production.

Fluctuations in temperature, or sudden changes in the material being fed to the digester, can cause a microorganism imbalance. Foaming can occur due to too little, too much or inconsistent sludge mixing.

Feed fluctuations may cause an imbalance between acid-forming microorganisms and gas-forming microorganisms. The expanding mat of foam from the upset digester can overflow the tank, requiring significant maintenance and cleanup. Taking a digester out of service, without advanced planning, can lead to significant process control issues and the remaining digesters will have to take on the extra load.

Finally, a “sour” digester can cause odours that the community will be quick to report.

The potential of digester tank upset has made TPAD seem risky. However, the benefits can be achieved with little risk. With proper instrumentation and controls, plant operators can be alerted before upsets occur, allowing them to take action and avoid process upsets.

Gas flow meters typically used in biogas production are slow to respond and may not report minute shifts in flow. For the system to be fully optimized, equipment must respond quickly and reliably. Meters which respond quickly to very small changes in flow can provide the response window necessary to avoid upsets.

Biogas has varying flow rates and contains a great deal of moisture. Moisture in the gas lands on the meter’s thermal sensors. Designed to measure vapour, liquid will cause the sensor to give false flow readings. Up to now, the ability to use phased digestion accurately and effectively has not been possible, simply because digester gas could not be measured with confidence. Most flow meters do not provide both rapid response time and accuracy.

Case study

One regional facility treats 230 MLD of wastewater. In addition to local sewage, it also receives deliveries of organic material from local industries and agriculture. To keep up with demand, a state-of-the-art TPAD system was installed.

A 454FTB WGF unit was installed to measure output from the acid fermentation tank.

However, when minerals clogged the heat exchanger to the acid fermentation vat, the temperature rose, causing a bloom of bacterial growth and an increase in biogas production. Before operators had any warning that something was wrong, the digester was upset. Foam and other digester contents overflowed and caused the digester lid to tilt into catastrophic failure.

Operators were unable to effectively control the TPAD process. The plant manager heard about Kurz Instruments’ new Wet Gas Flow™ technology and installed a 454FTB WGF unit to measure output from the acid fermentation tank.

The sensitivity of the meter was demonstrated when plant operators noticed an unexplained spiking of flow signal. Investigation revealed the sensor was reading flow caused by bubbles coming up through the mat of foam. The efficacy of the meter was shown the next time an upset occurred. It saved the plant from failure by immediately reporting the increase in digester gas flow caused by a temperature increase. This allowed operators time to stop the runaway reaction and regain control of the process.

The Kurz Wet Gas Flow meter allows multi-phase digestion to be effective and reliable, which means wastewater treatment plant operators can increase biogas production and digest waste more completely with far less capital expenditure.

Ethan Webster is with KURZ Instruments Inc. This article appears in ES&E Magazine’s February 2018 issue.

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