At the heart of clean water
New technology is a necessary but costly investment
As Big Sky County Water and Sewer District (BSCWSD) general manager Ron Edwards pointed out at the June 18 board meeting, something called a membrane bioreactor is at the heart of most municipal water treatment plants.
“You have to know what’s at the heart of the treatment plant in order to design the rest of it,” Edwards explained.
The near-comic book terminology – membrane bioreactor (MBR) – is actually a complex and costly system, which is touted as being one of the most important innovations in wastewater treatment by the National Institute of Health. “An MBR is a hybrid of a conventional biological treatment system and physical liquid-solid separation using membrane filtration in one system,” a study titled, “Membrane Bioreactor (MBR) Technology for Wastewater Treatment and Reclamation: Membrane Fouling” from the Department of Civil Engineering at the University of Calgary stated. The membrane has a dirty job.
“It’s basically putting a really, really, tight membrane down into the soup of wastewater and microbiology, and drawing water through the membrane while blocking solids. So, the membrane will foul (plug) with everything that it is in the soup. The MBR system uses back-pulsing, membrane rests and air scouring to fight the fouling. Eventually the membranes must be cleaned with acid and sodium hypochlorite (bleach). You’re always working to minimize or slow the fouling of the membrane, so you pull more water out of it. Your flow rate stays consistent because you have so many membrane modules in operation at any given time, but they do foul,” Scott Buecker project manager with engineering company AE2S explained in later conversation.
Buecker said to picture a thick, porous white balloon. To the naked eye, the membrane looks like a strand of rubber.
“The ones we are looking at for Big Sky are made of PVDF (polyvinylidene difluoride) and are just a specialty plastic. They hold up well in wastewater in all kinds of conditions, with proper care,” he said.
The wastewater industry characterizes membrane fouling as reversible or irreversible.
“Reversible happens all day, all the time, with biological, chemical and physical substances,” he said. The battle to keep the membranes clean is constant and relies most heavily on air scouring, which is just passing air bubbles across the surface. Eventually, the membrane will require a CIP, industry lingo for “cleaning in place” – a thorough cleaning of the membranes with chemicals like citric acid, sodium hydroxide, or chlorine.
“All fouling is not created equal – sometimes it’s biological, sometimes it is chemical, like scaling, and sometimes it is physical,” Buecker explained. Wastewater treatment operations are constantly battling fats, oils and greases or FOG. These substances can cause permanent fouling of a membrane.
“That gets expensive,” Buecker said. “You can’t run fats oils and greases up against the membranes, or it will foul them and can be very difficult to remove. When we use MBRs we try to get all that FOG out upstream, by trapping it and removing it with skimmers– very similar to how the oil and gas industry deals with it.”
Big Sky currently has oils and fats issues in the collection system.
“It’s a big issue in any part of the country, but in Big Sky I think what is happening is that you have more restaurants proportional to the number of people that live there, than the typical community,” he said. “The District is working towards a program to better monitor that and encourage restaurants to maintain their grease traps better. Hopefully we won’t be dealing with it as much as they deal with now.”
Further complicating treatment at the future facility is that based on historical data Big Sky has staggeringly high phosphorus in its wastewater, 2-3 times greater than what Buecker usually sees in municipal wastewater. The state of Montana is technically one of around 25 states in the country who have said they want phosphate free laundry detergent. There is no real enforcement mechanism in place, so the use of detergents containing phosphate exists in a gray area. Dishwashing detergent is a different beast, especially at the commercial level. Phosphate detergents are known to produce crystal clear glass wear, especially when the water is somewhat hard, he said.
“If we can cut down how much is in the wastewater, we can cut down on how much chemical we have to throw at it to remove it to extremely low levels,” he said. Truckloads of the chemicals needed to adsorb phosphate will be brought in from elsewhere, which is costly and has an environmental impact. And the river will still be impacted, because the chemicals utilized to reduce phosphorus concentrations leave behind sulfate or chlorides, which raises the total dissolved solids – “salts” – that go into the river, Buecker explained. While discharge of these low level salt concentrations is less harmful to the river than phosphorus, it would be better to mitigate the phosphate use in the community before it reaches the treatment plant.
“We just want people to get smarter about it and maybe look at what detergent they’re using, and if contains significant amounts of phosphorus, find an alternative,” he said.
Competitive bidding between the primary MBR suppliers resulted in costs for the MBR equipment being less than budgeted, however there is a difference of nearly $170,000 in engineering and support services or “soft costs” exists between the companies.
However, a detailed analysis of the installed costs and competitors’ bid responses indicate that the overall value of the two lowest bids is very similar.
“An evaluation of the bids resulted in $1,807,930 for Suez and $1,832,746 for Evoqua,” Buecker explained.
Board member Scott Wheeler inquired as to which company would be the better fit. Evoqua appears to have budgeted to provide a lot of help, Suez did not.
“I’m biased, I’m going to want the team that budgeted for significant support versus the team that cut it close,” Buecker said.
Board member William Shropshire said it was an interesting analysis.
“I think this is remarkable that we’re that close,” he said.
Edwards said both companies are very reputable.
“They’re close enough to where we can select one or recommend one for selection,” he said.
Wheeler said the closeness in cost is a credit to Buecker.
“It’s so tight that you obviously had a tight RFP (request for proposal),” he said.
Buecker reiterated that the equipment is solid for both companies. He said it will come down to what the board believes will benefit them long-term in terms of support.