How it's done, how you can help
A lot of money and energy goes into wastewater treatment, and one way for us to cut our collective costs in treating wastewater is to not pump as much of it into our sewers in the first place. Recycling ‘gray water‘ (used water that isn’t contaminated by human waste) is one obvious way to cut back on wastewater, while cutting out certain unhealthy habits around what we throw down the drain will help the health of the waterways our treated (and sometimes untreated) wastewater flows into.
The modern sewage treatment plan
A typical wastewater treatment plant treats sewage in stages, and combines physical, chemical, and biological processes to separate out solids, oils, and sediments, then to break down organic matter into a form that can be flocculated out of solution, and finally to sterilize the water to remove any pathogens. Let’s look at each of the stages in a little more detail, as it will help us understand how what we do in our own homes affects how much money and energy it takes to operate such a wastewater treatment plant.
The first step, sometimes called pre-treatment, involves removing any solid objects that are either damaging to later stages of wastewater treatment, or are easily removed by mechanical means. For example, the wastewater typically throws through a screen that can collect solid objects such as branches (typically washed in from storms), plastic bottles, bags, and other plastic waste, and other solids that tend to be washed into storm sewers during rains or snow melts (much of this screening is only required in systems where sanitary and storm sewers are commingled – it’s hard to flush branches or tin cans down the toilet!). Pre-treatment can also remove solids that were flushed down a toilet and probably shouldn’t have been – dental floss, sanitary napkins, rubber duckies…
Another part of pre-treatment involves allowing the water to slow down in a large retaining area where particulate such as sand, dirt and grit can settle out – as with the screening of branches and other storm sewer waste, this is more important in combined sanitary / storm systems, but less important for sewers that are strictly for sanitary sewage.
Fats and oils, which either naturally rise to the surface or are helped along by aeration (bubbles) from below, may also be skimmed off the top of this same retaining area, or of a second retaining area specifically for that purpose.
Primary treatment continues the wastewater treatment process of allowing solids and fats to separate out, except that the solids tend to be in the form of sludge from much smaller particles suspended in the water. This sewage sludge is continually scraped off the bottom and is later treated separately.
What to do with this sewage sludge presents a major challenge in waste disposal.
Primary or sedimentation wastewater treatment does help clean the water, but we mustn’t forget that at the end of the wastewater treatment process there is not just clean water but all the byproducts that are removed.
Other ways in which the sludge is disposed of are to break it down using aerobic or anaerobic bacteria, or to combine it with carbon-rich materials such as wood or paper wastes and compost it. The end product can then be applied as fertilizer to agricultural land, although some of the same concerns that apply to pellets will apply here. Finally, sludge can be landfillled without first being treated, which if the landfill is properly built will prevent the sludge from contaminating the surrounding water table, but is still a concern from a sound land use perspective.
Secondary treatment is where bacteria, yeast, and other microorganisms get to work breaking down dissolved fats, sugars, soap and detergent residues, and other human and household sewage waste. Some of the waste is converted by the microorganisms into gases (carbon dioxide, methane, nitrous oxide); the methane is typically burned either as waste (to avoid releasing it into the atmosphere, since methane is a much more potent greenhouse gas than the CO2 and water vapor that are released by burning it), or to generate electricity at the same time. As the microorganisms break down these nutrients, the resulting waste organic matter tends to clump together in a process known as flocculation. This ‘floc’ then settles to the bottom of the sedimentation tank and can be removed from below. At the end of secondary treatment the organic content and percentage of suspended solids in the wastewater are both much lower.
A number of different processes may be applied as part of secondary wastewater treatment to further promote biological action and settle out floc.
Tertiary treatment usually involves one or more forms of physical, chemical, or biological filtering to further clarify the water, for example passing the sewage through sand to remove the rest of the sediment, or through activated charcoal to absorb toxins (which should probably not have been in the sewage in the first place). Tertiary treatment also has to deal with the excess of two particularly problematic chemical nutrients that are found in sewage, namely nitrogen (urea, found in urine, as well as other nitrogen sources from detergents) and phosphorus. These chemicals have to be further broken down by a combination of biological and chemical processes, since releasing them directly into a waterway causes algal blooms (the nitrogen and phosphorus stimulate algal growth to the point where the algae consume all the available nutrients and die en masse, leading to decomposition and the loss of most of the oxygen in the water, which can also wipe out much of the animal population (zooplankton, invertebrates, fish etc. in the water). Blue-green algae, one type of algae that enjoys this feast of nitrogen and phosphorus, also produces toxic chemicals that contaminate the water.
The final stage of wastewater treatment before release into the environment is disinfection, which is necessary because much of the sewage started out as human or animal waste, which contains dangerous pathogens that could contaminate our waterways. The three main treatments to disinfect wastewater are to apply chlorine, ozone, or ultraviolet light.
Chlorine is most commonly used – just as it is the most common way we disinfect drinking water. Chlorine kills the bacteria at relatively low concentrations, although unfortunately it also produces toxic byproducts in the disinfected water, and must be removed since the chlorine itself is toxic to the ecosystems the treated water is released into.
Ozone also kills pathogens and does not require a follow-up process to remove the ozone since it naturally breaks down to oxygen molecules, but it is a more expensive process and so rarely used.
Ultraviolet light works well if the treated water is pretty much clear, but cloudy water can make ultraviolet light ineffective by shielding pathogens from its destructive powers, and again its higher cost puts it at a disadvantage over chlorine.
Storm sewage treatment
Storm sewers in larger municipalities are kept separate from sanitary sewers, because many of the processes used to treat sanitary sewage are unnecessary for storm sewage. In smaller municipalities storm sewers may drain directly into waterways. In larger municipalities the storm waste may be treated using some of the sanitary sewage techniques (screening, sedimentation ponds) to remove solids. In cases where storm water flows into sanitary sewers (for example, household roof downspouts connected to a sanitary sewer), rainfall can create challenges for the sanitary wastewater treatment system, as after a heavy rain the sanitary sewer treatment plant may be overwhelmed and some sewage (commingled storm and sanitary sewage) may spill into waterways untreated.
How we can lower the ecological impact of sewage treatment
From this description of wastewater treatment we can see a number of opportunities for reducing the environmental impact of our home water use. In terms of pre-treatment, we can avoid contaminating the water with sand, grit, materials washed into the sewers during storms, and household materials or objects that belong in the garbage rather than the sewers. For primary wastewater treatment, disposing of fats and leftover foods through a municipal composting system is more efficient and environmentally sound. For secondary treatment, again reducing the organic input to wastewater treatment systems can be accomplished by household composting. For tertiary treatment, reducing our use of phosphorus in household cleaners will help.
Here are a few key guidelines for a greener approach to wastewater treatment, elaborated below:
- Don’t use the sewer as a garbage disposal
- Keep toxic chemicals where they belong
- Reuse wastewater as gray water where possible
- Keep storm and sanitary sewers separated from cradle to grave
Don’t use the sewer as a garbage disposal
Your toilet will happily flush down many small things – such as dental floss, used Kleenex, crumpled paper, plastic wrapping – that add a burden to wastewater treatment plants. Dental floss, for example, can slip through the pre-treatment screens and get tangled in pump shafts, causing serious damage, while extra facial or bathroom tissues are harmless but increase the burden on wastewater treatment. The only thing that should go down your toilet is really human waste and bathroom tissue, and a modest amount of water (meaning, try a low flush toilet!) to move it all down.
Out of doors, it might be tempting to use your garden hose as a broom, pushing dirt, trash and yard waste off your driveway or sidewalk with a spray nozzle and moving it towards the street gutters. But all that stuff winds up down the storm sewers and requires some level of treatment to remove. When you dispose of such items by picking them up or sweeping them up, you can separate out the biodegradable, recyclable, and disposable materials, and send each to its own place. When you wash things down the storm sewer, they all wind up in one place and cannot be separated, so they all wind up going to the landfill.
North Americans throw out a huge percentage of the food they buy. Our ever larger refrigerators, and the ever greater distances between home and supermarket, mean we tend to do one big shop a week and then forget half of what we bought at the back of the refrigerator. Then we toss out what went bad since the last time we checked the fridge. We cook too much food and wind up tossing out half of what we cooked.
It seems criminal to pay farmers to grow food, pay truckers and distributors and retailers to get it to us, and then to throw half of it into a landfill. Some people assuage this guilt by chucking the unwanted food down the drain instead of into the landfill. I’m talking about waste disposal units, which are really not as great for the environment as they sound. When you compost food in your backyard composter, vermicomposter, or in a municipal compost collection scheme, you are keeping the compostable materials separate and allowing them to be broken down into reusable compost efficiently and cost effectively. When you grind food up and send it down the drain you are adding to the burden on the sewage system. In-sink disposal units are a great convenience and a really bad idea. They are even banned (from new home construction or kitchen remodels) in many areas, because of that added sewage burden.
Keep toxic chemicals where they belong
Used motor oil: People get upset about oil tankers running aground, but did you know that every year more oil gets into our waterways from people doing their own oil changes, and from oil dripping from poorly sealed engines, than from a spill the size of the Exxon Valdez? (The BP oil well blow-out in the Gulf of Mexico, of course, was a much bigger deal.) Sewers are no place for used motor oil. Many of the toxic chemicals in oil are not removed by the wastewater treatment process, and wind up in our lakes and rivers. If your engine is leaking oil, get it fixed. if you change your own oil, take it to a service station or toxic waste collection site for recycling.
What’s even worse about the way some people dispose of their used motor oil down the sewer is which sewer they toss it down. People don’t want to stain their toilets or laundry tub with blackened Quaker State, so some people dump used oil down a storm sewer at the curb instead. Remember, in sanitary sewage treatment, there is a skimming step that removes grease and oil from the surface. Not so in most storm sewer treatment plants – the treatment consists largely of screening out large debris and letting sediment settle to the bottom. Pouring your oil down the storm sewer is worse than pouring it down the sanitary sewer, although doing either is illegal and just plain wrong!
Prescription drugs: believe it or not, drugs you toss down the drain (and some of the drugs you consume, passed out unaltered in your urine) wind up in our drinking water! Our wastewater treatment systems aren’t able to completely break down many medications, and when you dump unused medication down the drain (or it passes through your body), much of it can wind up back in the water supply and get pumped back into your tap water. This is especially a problem in arid regions where wastewater treatment includes recycling the treated water directly back into the water supply; detectable levels of estrogen from contraceptive pills have been found in drinking water from recycled wastewater sources.
Water-based paints: People prefer to use latex or acrylic paints over oil or alkyd paints because water-based paints give off fewer fumes, but also because the paint can be cleaned off brushes, rollers, and one’s skin with water. That’s definitely a convenience, but that doesn’t mean you should toss the paint wash down the drain.
A couple of days ago one of my neighbors came out to the curb with a 5 gallon bucket, and dumped a white watery liquid into the storm sewer. I’m pretty sure it was water mixed with white latex paint from cleaning brushes and rollers. This is absolutely the wrong way to deal with latex paint waste. While our wastewater treatment plants can handle a little rinse water from washing paint brushes, we should try to minimize the amount that goes into our sewers. Here’s what I recommend (having been a student painter for two summers, I actually know a thing or two about this):
Start with an empty five gallon bucket where you can gather the combined water and paint waste.
For brushes, get as much of the paint off as possible onto walls where it’s needed, or onto newspaper. Squeeze the brush into a rag to get even more of the latex out. Mix a small amount of water and dish detergent in a yogurt tub, and repeatedly press the brush into that. Work your fingers into the base of the bristles to dislodge paint that’s trapped in there. Pour this paint water into the five gallon bucket. Repeat if necessary, then brush the paint brush on a piece of newsprint. Only then should you wash the brush in the sink.
For rollers, if you are stopping for the day but will be painting the same color a day or two later, wrap the roller in plastic wrap or place in a plastic bag, ensuring the entire roller is covered and has no air exposure. Store in the refrigerator. Remove it from the refrigerator about 15 minutes before you need to use it again.
If you’re done with a particular color, one option is to just throw out the roller. This saves our wastewater treatment plants from a lot of toxic waste. But I don’t know many people willing to shuck out $4 or $5 on a high quality roller only to toss it out after painting one ceiling or four walls, and buying a low-quality roller usually means a low-quality paint job.
If you’re determined to reuse the roller, first roll as much of the paint onto a dry wall of that color as you can (or onto old newspaper if you don’t have an available wall of that color). Then use a stir stick from the paint store to squeeze the extra paint back into the can. You may have noticed that one section of a paint stick has a curve in it; you can hold this level with the top of the paint can and scrape the sides of the roller with it, while pulling the roller up, to push paint off the roller into the can.
Only after you’ve gotten as much paint out of the roller as possible should you wash it. Do a first wash in an empty 3 gallon bucket to remove the bulk of the remaining paint residue, and pour that into the 5 gallon bucket that holds the paint water from the brushes.
For roller trays, one option is just to let the latex paint dry in the tray and become a new bottom to the tray. This works for a while, but eventually there will be so many layers that bits of paint will flake off and get into your fresh paint. A better option is to use a tray liner (you can buy them at your paint retailer) and throw it out when you’re done with that color.
And what about that five gallon bucket that’s now full of water and paint? One option is to pour the contents into empty one-gallon paint cans, and leave them, with their lids off, in a place where the liquid can evaporate. I leave paint water in cans in my bicycle shed, and after a month or two all that’s left is a half inch or so of dry white gunk at the bottom; I then toss the paint can into the trash (it’s not recyclable if it’s that full of gunk). Or you can let the liquid sit for a few days, at which point the upper layers will clarify, and you can water your garden or lawn with that, and leave the lower layers to dry out as described.
Reuse wastewater as gray water
My grandfather, who began his career during the Great Depression, was so frugal that after my dad and aunt and uncle had their Sunday bath (a decade later during World War II), he would run a garden hose from the second floor bathtub, syphon off the used bath water, and water the Victory garden. Water, like everything else, was too scarce and expensive to use only once.
When water is used for activities like bathing, washing dishes, washing laundry, or mopping floors, it is obviously too dirty to reuse for the same task, and certainly too dirty to reuse for cooking, but it’s often still clean enough that it can be used for a task that doesn’t require the water to be quite as clean, such as watering plants or flushing toilets. Water from one task that is saved for reuse on another task is called gray water (often written as one word, graywater) and the more creative you get at finding extra uses for your gray water, the less water you’ll use (so you’ll save on your water bill) and the less burden you’ll impose on your wastewater treatment plant.
One of the easiest ways to take advantage of gray water is to reuse the water in your kitchen sink, in place. When I wash vegetables or leafy greens before cooking a meal, I put cold water in a clean sink, rinse the produce, then remove the produce to dry, dice, chop, or whatever. The water, now a little dirty or gritty, stays in place. Once I’m done using the pots and pans for cooking, I use that same water to rinse off grease and other food residue before I drain the sink. I then do the pots and pans in a fresh sink of hot dish water, and I leave that dish water in the sink so that when we’re done eating, we can rinse our plates and cutlery in the dirty dish water, before putting them into the dishwasher. Since our dishwasher has a ‘Smart Wash’ feature, which uses less energy and fewer wash cycles if it detects there is less food residue in the dishes, this cuts back on my dishwasher’s energy and water use.
A couple of other good uses of grey water are to reuse dish water to mop your floors, and to save pasta water to prerinse dishes. If the dish water isn’t too greasy (which it won’t be if you followed the earlier technique of saving your produce rinse water to prerinse the dishes), you can first filter out any food solids with a hand sieve, then use the sink as a mop bucket. And if you’re straining pasta, put the colander into a large pot and strain the water into the pot; it’s very hot and will do a good job of prerinsing dishes. You can do one better when boiling corn on the cob: just pour the corn water into the sink with the plug in, and use that for washing dishes.
If you want to get extreme about it, you can go country and remove the P trap or S trap under your kitchen sink and install a bucket instead, or better yet you can replumb your sink so that it drains to a rain barrel outside, for use in watering. (Be sure to filter the water going into the rain barrel or it will soon silt up.)
In the bathroom you can buy contraptions that capture the water from the bath or shower drain and pump it into a holding tank above the level of the toilet tank. As the toilet tank contents are flushed out, the toilet tank can then automatically refill from the gray water tank (if it has water in it) before calling on fresh water to top it up.
Another way to reuse gray water is when doing laundry in a top-loading washer. (Of course, you’d be better off switching to a front loader.) When I was a kid our washing machine had two hoses that drained into two side-by-side wash basins. Wash water drained into the left hand sink, and rinse water drained into the right hand sink using a hose that went right to the bottom of the sink. My mother would plug the rinse water sink, so the rinse water was captured there; on the next load, the washer would suck in the rinse water through that same hose and use it for the wash water.
One concern about gray water reuse is the question of contaminants in the gray water reaching public waterways or storm sewer systems. Household gray water often contains soaps, detergents, or food byproducts. While there is little harm in most cases in watering your garden with gray water, if the soaps, detergents or food byproducts drain off your soil and flow into a waterway or storm sewer, the effect is to add to the nutrient load of the lake or river the gray water eventually drains into. This is why it’s better to pass the gray water through a rain barrel, where sediments will settle out, and to filter it before use. In drier regions where gray water recycling is commonplace, such as Arizona, there are regulations that require sedimentation and filtering of gray water prior to application to surface plants.
How we benefit from collective action on wastewater management
What’s in it for me, you ask, to worry about where my wastewater goes and how it’s treated? It’s true that if you act alone and reduce the amount of wastewater you send down the sewer – or are careful about what you put into it – you won’t suddenly see your water bill drop in half. But as more and more people become conscious of wastewater treatment issues and how to avoid the worst offenses, our treatment costs will fall, our water use will decline, and the lakes and rivers at the tail end of the process will be cleaner and safer for everyone. Taking a principled stand on wastewater treatment isn’t about personal gain, but about building a better community.
To learn of other home water saving tips, check out our water saving guide article. Also, if you’re interested in becoming an active participant in protecting the North Branch watershed, join the NBWA today!