How the treatment plant works...
As Sewer Superintendent Brendan O'Regan points out, the treatment of wastewater at the plant involves both a physical-chemical and a biological process - as well as a lot of recycling.
"Things are constantly in motion," O'Regan says about the way the wastewater is recycled continuously throughout the different treatment unit operations. "As that flow is sent there, this flow is sent here. As flow comes into the plant, other flow is exiting. It's a dynamic process."
Taking a tour of the plant with O'Regan last week, the superintendent explains that the process begins as the wastewater enters what is called the headworks building. Upon its arrival to the building, the wastewater is treated with ferric chloride, which takes the initial rotten-egg smell out of the wastewater caused by hydrogen sulfide, O'Regan says.
Also in the headworks building, a piece of equipment called a muffin monster, removes undesirables that shouldn't be in the sewer system - such as soda cans, dirty diapers and pieces of wood - and grinds them up.
During this initial pre-treatment stage, a device called a grit chamber, which is somewhat self-explanatory, removes the grit from the wastewater such as pebbles and course sand. As the wastewater makes its way through the device, the heavier matter - or grit - settles to the bottom of the chamber, as the lighter stuff overflows through the top and makes its way to the next treatment unit operation.
The wastewater then makes it way to the "wetwell," which includes a bubbling gauge that monitors the pressure of the collected wastewater. Enough wastewater must be collected in the wetwell for it to be pumped to the next treatment unit operation. The higher the wastewater level collecting in the wetwell, the harder it is for the gauge to blow the bubbles to the surface as a result of the increased pressure. When the pressure gets to certain point, a pump kicks on sending the wastewater to the adjacent pump room.
Upon entering the pump room, the wastewater goes through a big pipe called the influent meter, which measures the flow coming into the plant. The meter actually converts the pressure of the wastewater into a flow rate, O'Regan says, and the information is electronically sent to the plant's control room.
From there, the wastewater is pumped to a primary sludge pumping station, in which the second deodorization process takes place using the chemical sodium hypochlorite.
"Like when you're pumping anything, you can get splashing," the superintendent says. "When wastewater splashes, it can have odors, so we're treating the odors from this particular location with what's called a chemical scrubber."
After the second deodorization process, the wastewater is split and sent to the plant's two nearby primary clarifiers. While it may seem like a no-brainer to some, the reason why they're called clarifiers, O'Regan says, is because they clarify the water. Just as there are two primary clarifiers which treat the wastewater first, there are also two secondary clarifiers which treat the wastewater later on in the process. The clarifiers, which run at least 10-feet deep, are above ground and are perfectly round in shape. The secondary clarifiers are larger in diameter than the primary clarifiers.
It's at the primary clarifiers where the first real significant treatment of the wastewater takes place. "Let's say you put too much Nestle Quik into some milk," O'Regan explains the process in layman's terms. "If you're mixing it up, it will all be in suspension. If you stop mixing it, what happens? It all settles to the bottom. That's the premise of how this works. The heavy stuff that's in the wastewater settles to the bottom and the stuff that doesn't settle to the bottom overflows."
The heavier material that settles to the bottom of the primary clarifiers is then pumped to the "gravity thickeners." The small tank-like buildings allow the heavier solids more time to settle to the bottom and thicken. The heavier solids at the bottom of the thickeners are then pumped to the presses in the operations building. Here, as much water as possible is squeezed from the slurry and it's pressed into what's called filter cake, O'Regan says.
The filter-pressed cake then goes onto a conveyor belt, is loaded onto a truck and hauled out of the plant. The water pressed out of the sludge is returned to the headworks building where it starts the process all over again, O'Regan says, and is just one example of how the wastewater is recycled throughout the entire treatment process.
A local company called Agresource, mixes the filter-pressed cake with leaf and yard waste, turning it into compost, also referred to as "bio-solids," O'Regan says, "because you're not burning it and you're not throwing it into a landfill. You're actually recycling it and getting a beneficial use out of it.
"I know this may sound hard to believe and it's hard to say that a sludge is clean, but the sludge is so clean that you can plant vegetables in it for human consumption," O'Regan adds. "That's how clean it is. It meets Environmental Protection Agency standards."
A recycling cycle
As the heavier solids in the thickeners are sent off to the presses and shipped out of the plant, the lighter matter at top of the thickeners overflows back to the primary clarifiers - a second point of recycling. Meanwhile, the lighter material in the wastewater at the top of the primary clarifiers overflows to next treatment unit, the aeration tanks. It's at this point of the process, O'Regan says, where the process turns from chemical-physical to biological.
While the wastewater flows into the aeration tanks, it's also flowing into the secondary clarifiers, which perform the same function as the primary clarifiers, O'Regan says, where the heavier material settles to the bottom.
However, rather than being sent to the thickeners, around 30 percent of the heavier material at the bottom of the secondary clarifiers is sent back to the aeration tanks, indicating a third point where the wastewater is recycled.
Of that 30 percent, a small portion, roughly 1 percent to 3 percent, is sent to the sludge holding tanks at the plant. Called "waste activated sludge," the material remains in the tank for about 10 days, where it is broken down by air. In a fourth recycling process that O'Regan calls "co-settling," the broken-down sludge overflows to the primary clarifiers, where it settles with the material already at the bottom. The mixed sludge is then pumped to the gravity thickeners and eventually the presses before entering the world as a bio-solid.
Meanwhile, the material being pumped from the secondary clarifiers to the aeration tanks has a purpose all its own. The material, also called "return activated sludge," is mixed with the wastewater overflowing from the top of the primary clarifiers. The return activated sludge is actually bacteria and the wastewater is a constant food source for the bacteria.
"The bacteria eats the bad stuff in the wastewater that comes from the primary tanks," O'Regan explains the biological process. "Essentially what it does is it turns that bad stuff into water, carbon dioxide or more bacteria like themselves."
Called "endogenous respiration," the process calls for oxygen. "Bacteria is a living organism. Like us, bacteria needs oxygen to live so we need to provide oxygen in these tanks in order to promote that biological process of bacteria eating the bad things in the wastewater," O'Regan says. "So how do we create that? We have big spinning disks on these tanks that spin air from the atmosphere into the wastewater. That's why it's called aeration, because it is essentially mixing and throwing air into it to allow the bacteria to eat the bad stuff in the wastewater."
Toward the end of the process, O'Regan points to the cloudy wastewater flowing into the center of the secondary clarifiers. However, as it overflows along the edge of the secondary clarifiers into the chlorine contact chambers, the wastewater looks notably clear, since it's had time to settle out.
It's not uncommon to see ducks swimming in the secondary clarifiers, O'Regan says, which was the case during last week's tour. "It's clean enough for the ducks to be there," he says.
Upon entering the zigzagged-shaped chlorine contact chamber, the wastewater is treated with chlorine to kill as much remaining bacteria as possible. The process takes about 45 minutes, O'Regan says, after which the chemical sulfur dioxide is added to eliminate any remaining chlorine that wasn't used up in killing the bacteria. The remaining chlorine entering the river, also called chlorine residual, must not exceed .3 milligrams per liter, the superintendent says, "which is interesting because they're people upstream from us that can have higher chlorine limits than we do. I don't quite understand why, but that's the case."
Using a sample taken from the end of the contact chamber, a device called a chlorine residual analyzer tells plant employees how much chlorine is still left in the treated wastewater in the chamber. That way, the appropriate amount of sulfur dioxide can be added to the treated wastewater so that the chlorine residual meets acceptable limits.
"It's very important to realize that this meter is monitoring the residuals before the dechlorinating agent is added," O'Regan stresses. "We use this for guidance. As far as what we report to the state and to the Environmental Protection Agency (EPA), we physically grab the sample and bring it into the lab and do the analysis. When we installed this 18 months ago, we weren't required to install it. We did it on our own."
As part of a new National Pollutant Discharge Elimination System permit from the EPA, the agency could potentially require that the sewer plant have the chlorine residual analyzer anyway. However, the agency has yet to issue a final draft permit for the plant.
"They basically said, 'That's a good idea, we're going to make you do it now,'" O'Regan says. "We're just trying to show some initiative that we just don't wait around for the Department of Environmental Protection and the EPA to tell us we have to do stuff. We do it anyway."
After being properly dechlorinated, the treated wastewater is then released to the river. "Sometimes we have to go against the tide. That's why we have effluent pumps to pump the wastewater out there," O'Regan says. "But most of time, it just flows by gravity out to the river."
As for the odors that are sucked out during the process, they make their way through a large gray pipe at the plant and eventually a big mulch pile, also called a bio-filter.
"Essentially, the odors are basically treated the same way we treat the wastewater - biologically," O'Regan says. "There's naturally recurring bacteria in that mulch pile and as the air that we're pumping out pushes through the mulch pile, the bacteria eats the bad things in the air. We call it stripping because it strips the air."
Meanwhile, using samples taken from the treatment system, various tests are conducted in the plant's laboratory on a regular basis to make sure the treated wastewater meets acceptable limits under its permit.
"The plant depends on what I give them for results," says lab technician Steve Harris. "So everyday, I do the same tests and if I find that a number goes way up, or something's not right, I'll retest it. I'll let them know that I'm retesting it to make sure that something didn't go wrong. So you're always staying on top of that."
Harris says he takes his job very seriously, especially when it comes to taking and testing samples.
"I've been doing this for 20-odd years," he said, adding anyone can check his results anytime they want. "I have to do a state test at least once a year and I haven't failed one yet."
In the past, Salisbury officials have pointed the finger at the plant for being a big pollution generator. Others have gone so far to suggest questionable laboratory data. However, Harris stands by his test results.
my job. If someone came in and found out I was falsifying the test results,
I could go to jail and I'm not taking that chance," he says. "My
uncle lives in Salisbury and I've told him, 'Don't believe everything
you read in the papers, because it's not true.'"
|(This article replicated online with permission of the Merrimack River Current.)|