How sewage wastewater is treated
Yes, it’s time for another “finish your lunch first or better still don’t start it” article. In this article we’ll take a brief look at what happens to that lunch once you’re entirely done with it and how that relates to green living.
Just about everything about our modern lives is so complex and our labor saving devices and systems have come at a huge cost to the environment.
Take the simple process of pooping and peeing.
If you’re out in the real boonies and nature calls, answering that call can be very straightforward – point (or squat) and shoot, or dig a hole, do the necessary, clean up (using toilet paper made from recycled content of course) then cover the hole. The more adventurous may choose to do without the toilet paper, using whatever nature provides.
Wonderful. Let’s leave it at that, you get the idea.
Start doing that in your local park or front yard in full view of neighbors and you’ll soon be hauled off.
I’ve covered the topic of humanure and composting toilets before as alternatives to conventional sewage treatment, but this is more to flag why we need to treat waste differently than we currently do. The best way to do that is to first understand how it is treated now.
I’ll skip the bits we all know about (the steps leading up to the flush) and we’ll pick up the story from there. This deals with sewage that isn’t treated on site, such as in the case of a septic tank or blackwater recycling system.
Wastewater, which includes sewage, is 99.9% water, with the remaining 0.1% generally dissolved or suspended materials. Around half of the water a typical city in the developed world uses, not including industrial activity, is returned to sewers from household toilets, sinks, showers and washing machines.
Getting it to the sewage treatment works
Do you know where your sewage is treated?
I wasn’t sure of my closest waste water treatment plant, so I had to look it up. Adelaide’s wastewater system serves the city’s population through more than 7000 kilometres (around 4,400 miles) of sewer pipes and three major wastewater treatment plants. The closest one to me is about 4 miles away! The sewage from our place doesn’t get there under its own, ahem, steam – it needs to travel via pumping stations.
Usually a raked bar screen is used to remove large items to minimize risk of damage to pumps and skimmers used during primary treatment. The sewage may also be directed through a channel at a controlled rate to allow for grit and other items to settle.
Sewage primary treatment
This usually consists of pumping the sewage into huge holding dams where solids settle to the bottom and oils and lighter solid matter floats. The “floaties” are skimmed off and the heavier sludge is scraped towards a chamber where it is pumped elsewhere for further treatment.
Sewage secondary treatment
This removes dissolved and suspended organic matter and while the process varies, usually is carried out with the help of micro-organisms who digest the organic material. As it is an aerobic process, air needs to be pumped into the water.
The micro-organisms may need to be removed from the treated water prior to discharge; so even more processing is required.
Sewage tertiary treatment
This involves filtering and disinfection prior to discharge into a waterway, or for reuse in watering gardens or parks. The filtering may occur through sand and common methods of disinfection include ozone, chlorine, ultraviolet light, or sodium hypochlorite.
But I’m missing something here – what happens to all the “floaties” and sludge I mentioned earlier that is collected during the primary treatment?
More liquid is removed the sludge and then through aerobic/anaerobic or a composting process, the resulting substance called centrate is typically reintroduced into the wastewater process. In more advanced systems, it may undergo further processing to become fertilizer.
What about the pre-treatment stuff that is filtered out? What happens to that? Given my own previous experience as a contract cleaner and seeing what people attempt to flush down toilets, I can only assume it winds up in landfill or incinerated.
So as you can see, what is such a simple function out in the boonies is an incredibly time and energy intensive exercise in the suburbs, requiring a ton of infrastructure and resources to make it happen and keep it happening – and bear in mind the above was only a brief overview.
I’m still amazed that all these processes used to occur in our own blackwater recycling system at our previous house. I can’t tell you what the energy savings were (if any) compared to normal sewage treatment, but it sure didn’t have to travel so far and all the waste generated from our household eventually wound up watering our own gardens.
Given the complexity of treating sewage wastewater, as part of our green living efforts we need ensure what we send down the pipes is what is meant to go down them and try to send less to be treated.
For example, the most common way of disposing of old pills and potions is down the toilet. Unfortunately, many wastewater treatment processes can’t remove the dissolved substances – these medications are winding up in our waterways. So what should we do with them? That will be a topic for another article. No, I’m not trying to keep you in suspense; I just don’t have a clue at this point :).
In terms of sending less wastewater for treatment; using dual flush toilets helps, as does using low flow showerheads and other simple water saving tips including reusing washing machine water to water your garden (this is called greywater) – you can even consider taking your pee outside. Believe it or not, your pee is precious!
Until such time that urine reclamation becomes commonplace (and it likely will, due to a looming shortage of phosphate) and humanure practices become more accepted, it’s just a case of us all thinking a little more before we flush or use water that winds up in the same waste treatment stream as our sewage.
If you are fascinated by the concept of humanure; i.e. composting human waste, check out my related article. It includes a link to download a free comprehensive manual – it’s the good poop on poo!
Green Living Tips.com
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