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Building Principles: Wastewater Treatment — A Primer
Over the years, we’ve peeked at some of the components to wastewater treatment, but we’ve never really taken the process apart to help explain what is really happening and what the objectives are. Shout for joy and dance a jig, because that time is now! No. Really! This is great stuff, and I promise to keep it clean.
The decomposition of digestive waste is as natural and nearly as old a process as our Earth itself. Otherwise, we’d all be up above our noses in dinosaur dung! But really: What happens and why? Spread over a large enough area, natural microbes would eventually decompose the waste completely and safely. Usually when a large volume of waste is concentrated in a small area, it becomes literally a toxic dump harmful to people, animals, and the environment. Modern wastewater treatment uses the same microbes. But by separating certain parts of the process and adding scarce ingredients, decomposition occurs much more quickly and in a very small space.
A multitude of different waste-consuming microbes exists naturally in the digestive system. In a symbiotic relationship, they help extract nutrients from the food that’s been eaten, while the host provides them a suitable home. Because that environment isn’t perfect for all of the different types of microbes, certain ones thrive and others just “make it by.” Once excreted into a new environment, though, different ones proliferate and others die off. Modern wastewater treatment plants encourage microbes to feast by providing each specialized group as close to a perfect home as possible. By manipulating certain aspects like pH and dissolved oxygen, we encourage certain groups of beneficial bacteria to flourish, and in so doing, push the waste decomposition process through the toxic phases toward inert compounds like nitrogen gas and carbon dioxide.
The processes are split essentially into two categories with different classes of microbes dominating in each (environments with oxygen or without). These opposite and competing needs require that we develop and nurture two separate environments. Microbes that thrive in anaerobic (without air) conditions do the lion’s share of solid waste digestion. Aerobic microbes generally feed on chemicals contained in urine. Wastewater treatment plant processes are arranged in that order, so our virtual tour will follow that path as well.
Anaerobic Microbe Processes
Throughout the treatment process, engineers use the physical and chemical properties of the waste to accelerate the digestion. But nowhere is that more obvious than at the very beginning of the treatment train. By the time that the waste arrives for treatment, dissolved oxygen is nearly zero, and conditions are perfect to separate the digestible suspended solids from the liquid in which they are suspended. Much like a snow globe, by allowing the fluid to sit quietly with as little mixing as possible, particles heavier than water (suspended solids) settle to the bottom. Particles lighter than water (usually fats, oils, and grease, or “FOG”) congeal and rise to the top. Anaerobic microbes thrive in this environment where they consume the solids, generating heat and accelerating the process. By simply allowing nature to take its course, the first step in detoxifying wastewater happens almost by itself!
So what’s important to know about this part of treatment? First, to settle solids, the liquid in the tank needs to be allowed to settle and shouldn’t be stirred up. That’s the principle behind sizing a septic tank or primary clarifier. Too much water through the system keeps things stirred and solids in suspension. Connecting buildings with their bathroom and kitchen fixtures to an existing wastewater system can overload this very delicate portion of the system. Not surprisingly, many wastewater system failures occur soon after new facilities are added.
Next, human waste contains millions more microbes than there is food to feed them. When they run out of food, they’ll go dormant or die. There’s no need to add packets of enzymes, because often their “digestive action” releases gases that keep the clarifier agitated. Unless settled to the bottom of the tank, these particles can upset the balance in downstream parts of the system where the aerobic microbes live.
Finally, the settling area of any system also allows FOG to cool, congeal, and rise to the surface. These are not digestible by natural microbes, accumulating and filling an increasing volume of the tank. The over-accumulation of FOG washes for¬ward and clogs other parts of the system. So when Mr. Septic comes to pump the tank, he’s supposed to be: a) Removing the solids at the bottom of the tank, and b) Removing the scum on the top of the tank. He is NOT supposed to be draining the liquid in the middle and leaving the other stuff.
Simple wastewater treatment plants, like septic systems, emphasize this first part — separation. After a septic tank, the soil provides the screening mecha¬nism where certain air-loving microbes can work, with the liquid eventually draining to the water table. But where the liquid is to be discharged to a creek, stream, or river, additional treatment is necessary so that the nutrient balance there isn’t upset. Wastewater treatment plants provide another set of conditions where those compounds can feed aerobic microbes.
Aerobic Microbe Processes
“All I need is the air that I breathe . . . .” The next part of the treatment system encourages air-loving microbes to thrive. In the simplest of the advanced treatment processes, ammonia (itself a product of biological consumption) is converted into nitrate in a process called nitrification. Unlike the separation process, which depends on physical proper¬ties of the waste, this nitrification is a biological process exclusively. Completely converting all of the dissolved ammonia requires that there be enough microbes, enough air, and enough alkalinity at just the right flow-through rate. For example, for every pound of ammonia nitrified, over four pounds of oxygen and over seven pounds of alkalinity (calcium carbonate) are required to keep the microbes’ diet in balance and keep the process working at maximum efficiency.
Air must be dissolved into the water in small enough nuggets that the bugs can take a bite. Alkalinity (in any of a number of chemical forms including sodium bicarbonate, lime, and milk of magnesia) must augment their diet by being stirred into the solution. And like any pet, the microbes need a place to live. This is usually floating plastic discs, gears or wheels, fist-sized rocks, or honeycombs of plastic, depending on the configuration of the treatment plant.
If all of that weren’t confusing enough, the activity level of the microbes depends on the water temperature. Warmer water temperatures increase their metabolism, the population increases, and more ammonia is consumed. Colder water temperatures have the opposite effect.
Clearly, this is a much more complex process than simply separating the solids and FOG from the stream. These are living entities which must be properly, continuously fed if they are to survive, let alone thrive, to work for us.
So what’s important to know about this part of the treatment process? Incomplete treatment can be traced back to several of the parameters we’ve discussed. If the microbes run out of alkalinity or air before they consume all of the ammonia, or if there aren’t enough microbes, unacceptably high levels of ammonia will remain in the discharge. The state-issued permit has been violated and fines may be issued. But what else can throw a wrench in the works? Suspended solids that carry over from the settling facility can act as food for other microbes present, but which usually don’t have anything to eat. With food, they compete for air, effectively suffocating the less aggressive, slowly multiplying nitrifiers. Pushing more wastewater through the plant can stir up the settling tank and upset the biology in the aerobic tank. A slug of cleaning chemicals, floor strip waste, or antifreeze can poison the microbes, killing them. The list goes on and on. Knowledge and experience are necessary to observe symptoms, interpret them, and take the correct, timely action to right the situation and get the plant working within its permit again.
The very natural processes that decompose our waste, as well as the lack of apparent moving parts at wastewater plants, can be misleading to the uninformed. Without a doubt, successfully operating and maintaining a wastewater treatment plant cannot be taken lightly or as a part-time effort or hobby. Regulatory agencies and environmental watch groups aren’t particularly interested in the reasons why a wastewater system is malfunctioning or that the source is a children’s camp. From their perspective, the biological stress on a receiving water body is the same regardless of the source. When the permit is violated, penalties are levied.
Properly operated, managed, and maintained, a wastewater treatment plant can receive concentrated waste and reduce it to safe, naturally occurring nontoxic compounds that don’t harm the environment. Understanding the bio-mechanics of your system is the very first step toward consistent, environmentally responsible performance.
Rick Stryker is a professional engineer with a particular passion for helping camps with infrastructure, planning, and regulatory issues. He can always be reached at firstname.lastname@example.org or 570.828.4004.
Originally published in the 2012 March/April Camping Magazine.