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COVID19 | Wastewater Treatment Plant

The impact of COVID19 lockdown on your wastewater treatment plant

Friday the 27th of March 2020 was a surreal day that we knew was coming, but that didn’t make it any easier to comprehend. The Taoiseach, Leo Varadkar  made the announcement, essentially putting the country on lockdown but for some named exceptions. “Our country is rising to this challenge, and I’m convinced we will prevail,” he said. It is inspirational to see what this country is capable of in the face of this crisis, shutting down tourism, sport, arts/cultural events and social gatherings. Not something we would have predicted, at the start of 2020! Some people are still going to work every day to keep food on the shelves, deliveries or continue production, and of course, the healthcare workers who are the real heroes fighting this war on the frontline. Our sacrifice of staying at home and working from home seems small compared to the effort the healthcare workers will continue to put in over the next few weeks.

 

Our company provides water, energy and wastewater designs and solutions, so you may ask, where does wastewater come into this? Well, it is not top of most peoples’ lists (understandably), but it is our job here at WEW Engineering to think about wastewater, which still needs to be managed. Also, water quality and emissions limits, while not the most important issue at the moment, should be upheld even in the time of a crisis. The EU holds a similar view relating to carbon reporting deadlines, upholding the April 30th deadline for firms to surrender emissions trading system (ETS). Therefore, authorities will not and should not be turning a blind eye to breaches of environmental regulations. So, the humble wastewater treatment plant (WWTP) will have to continue to efficiently and effectively discharge effluent in accordance with all EPA/Irish Water licencing and regulations.

 

Most WWTPs consist of microorganisms (biomass) which feed on the pollutants. Like most living things they thrive on a consistent and steady feeding schedule. They don’t like being starved, they don’t like being overfed and in general, they don’t like change very much. Now, in the current climate there are very few places that have avoided change so how do you look after your WWTP to make sure it will keep discharging effluent within spec and not cause any further headaches?

I will highlight a few points worth considering for any WWTP that has found itself with no wastewater to treat or with a drastically reduced loading. Having no or very little loading hardly seems like a problem, but when we all want to return to some type of new normal it would be a shame for any production line in the country to be held up due to the WWTP not working.

 

Worst-case scenario

The likely worst case is that after the WWTP being neglected for a few weeks all the biomass is dead and there is nothing in the tanks to remove pollutants. Therefore, either production is stopped, or you start to tanker wastewater away using an environmental services company. Neither of these options are economic or productive. The WWTP then must be reseeded with healthy biomass imported from another WWTP, reseeding will take 1-2 weeks depending on the process and how much biomass is imported. During reseeding the loading on the WWTP starts at 0% and increases by 5-20% per day.

However, not all the biomass will die at once; biomass which remove Phosphorus and Nitrogen are more sensitive than that which remove BOD/COD. If the population of nitrifying/denitrifying bacteria die the ammonia figures in the final effluent will be very high. Another indicator is the presence of nitrates. If no nitrates are present it means, there are no ammonia oxidising bacteria (AOBs) and you will need to import some seed biomass as a partial reseeding.

 

Best-case scenario

In an ideal world, lockdown restrictions will be lifted, and a facility would ramp back up to 100% production within a day or two and the WWTP would treat the wastewater to the required specifications as it had been doing before lockdown. The output of this production line would be hoping to go from 100% on 27/03/20 down to 0% the next day, then stay at 0% for at least 2 weeks before getting back to 100%. Production lines will be slower to get to 100% due to a wide range of issues around personal distancing in the workplace, demand forecasting and sales pipelines etc. Every site will be unique in this respect, but to look at the issue of wastewater treatment generally, let’s assume a proportional relationship between production output and loading to a WWTP. We are asking a WWTP to treat 100% load, then 0% load for at least 2 weeks and back to 100% load, this will not work.

 

Three options available to an operator of a suspended growth WWTP:

  1. Maintain WWTP at 100% loading
  2. Allow loading drop to 10-25% while keeping the biomass healthy
  3. Allow Biomass die and reseed when possible

Before we look at the options in isolation it is important to note that to give the biomass the best chance of reacting to your business needs there are a few basic items that should be addressed.

 

Dissolved oxygen

• Make sure the biomass has the right amount of air in the right places and make sure the DO instruments are reading correctly.

• Make sure the duty/standby changeover is automated on the blowers should there be an issue and the WWTP doesn’t get checked for a few days.

 

Recirculation pumps

• In certain processes, the biomass depends on cycling through different environments such as anoxic/aerated.

• If the WWTP is fitted with recirculation pumps the auto duty changeover should also be checked.

 

F:M ratio

This will be mentioned multiple times below but just to be clear and keep it simple, the food to mass (f:m) ratio is the ratio of food as Biological Oxygen Demand (BOD) in kg to the mass of the microorganisms in the process as Mixed Liquor Suspended Solids (MLSS) in kg.

 

Maintain WWTP at 100% loading

This option requires acquiring a solution to feed the biomass during the downtime. Acetic acid, sodium acetate and molasses are examples of chemicals used as a BOD source. The BOD concentration of these solutions will usually be several orders of magnitude stronger than the usual wastewater so to ensure the correct BOD loading in kg/day only a small volume of the solution is required via a drip feed or dosing pump. Most of the environmental services of chemical supply companies will stock these or other solutions which may have Nitrogen and Phosphorus at the optimum 100:5:1 ratio for biomass. After a few weeks on one of these solutions, your biomass will be able to pick up right where it left off as the f:m ratio hasn’t changed throughout the downtime.

 

Allow loading drop to 10-25% while keeping the biomass healthy

This option is more economical than keeping the WWTP at 100% as it is not necessary to import as much artificial feed but requires more management. The idea is to keep the f:m ratio relatively stable with a gradual reduction of no more than 10-20%. So the f:m ratio won’t change much, but the feed or the loading will drop significantly, therefore the mass of biomass or MLSS has to drop. Biomass must be removed from the system as waste sludge, this means there is less biomass to feed. Reducing the MLSS in the system will take several days depending on the type of WWTP in question. Then when it is time to get back up to 100% loading, biomass must be retained to build back up the MLSS. Depending on the system this will take 2-5 days.

This option can be considered the damage limitation option. You want to try to avoid losing the biomass and having to reseed the WWTP but the costs of importing feed makes it hard to justify keeping the loading at 100% or close to it. By reducing the quantity of biomass in the process, the contents of the balance tank could be used to drip feed the process for some time and minimal if any imported feed would be required. The loading may also be much lower for some time, if production drops to 10-25% for the foreseeable future the WWTP may have to deal with that even though it was not designed to. Depending on our client’s requirements we will design a WWTP with a turndown ratio of up to 8:1 meaning the WWTP will treat wastewater nearly as efficiently at 12.5% load as at 100% load. Not all WWTPs can sustain being turned down to this level but usually minor changes can increase the turndown ratio significantly.

 

Allow Biomass die and reseed when possible

This option is hard to advise but may be a reality for many businesses especially the smaller WWTPs associated with tourism, sport and other facilities which won’t have the same footfall for some time. This may seem a cheap option in the short term and maybe the only option for some but after the WWTP is shut down and it comes time to start it up again seed biomass will be imported from a healthy WWTP. The start-up time and costs will be significant if starting from scratch, to try and reduce these we would advise certain pieces of equipment to be left running to avoid any damage. The aeration system if using piped air should be left running at a reduced rate to avoid liquid ingress and to avoid septicity. Mixers should be operated intermittently to avoid solids setting and going anaerobic which can lead to hydrogen sulphide emissions. Also, scums can create hard floating layers or foul instruments if mixing is turned off completely. The process of reseeding can be complicated especially with more advanced wastewater treatment processes such as Nitrite shunt, Enhanced Biological Phosphorus Removal or Annamox.

There is a lot to think about during these unprecedented times we live in. Hopefully, these options can help avoid issues with WWTPs, and give some guidelines for when production facilities are up and running again at full capacity. Future production could look very different as restrictions are gradually lifted, which could require WWTPs to operate in a new capacity with different organic loading requirements.

 

Wastewater Treatment Services

We will be working from home for the foreseeable future and visiting sites only where completely necessary, as per government guidelines. WEW Engineering are members of ACEI and Engineers Ireland, we are offering free consultations concerning the issues above or any other issues relating to Water, Energy and Wastewater.

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