What Really Happens to Your Shit

Apparently, we all excrete around 0.9 kg of poo per day. If you do the math, for a global population of 7.4 billion people, that’s 6.66 billion kilograms of poo everyday! That value doesn’t even include the amount of pee urinated everyday. So, where does all that shit go?

For people with access to toilets, the poo and pee goes down to the sewage lines. But, those sewage lines are only so large and cannot contain an infinite supply of human waste. To accommodate our waste needs, the sewage lines are actually connected to a sewage treatment plant (STP) located near the community. However, the treatment isn’t so simple. Because you eat and drink different things everyday, your waste composition changes daily. To figure out what really happens to your shit, BOX checked out the Manila Water Olandes Sewage Treatment Plant, which covers the following barangays: Cinco Hermanos, Industrial Valley, Sitio Olandes, Blueridge, and Libis.

The Olandes STP processes 10 million liters of combined household wastewater and drainage per day. Household wastewater includes human waste, while drainage includes wastewater from sinks, showers, laundry machines, and road drainage. Depending on where you live, your waste may require extra effort from Manila Water or any STP operator to reach the STP. If your location elevation is lower relative to the STP, a lift or pump station lifts the effluent (AKA sewage) until its elevation is higher than the STP’s. Then, the effluent flows to the plant by gravity. The lift station is used for locations near the STP, while the pump station is used for waste that needs to travel longer distances. If you’re already located on a higher elevation, then the waste flows down by gravity without any additional effort.

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Simplified sketch of the screens! See the pile of hair, dirt, and a Cream-O’s wrapper!

Once the effluent reaches the 2 hectare (which is apparently small) underground Olandes STP, it is processed through a coarse 0.3 mm screen and further processed through a fine 0.12 mm screen. At this point, the stench of the effluent is really strong and the effluent is black. The screens have to be cleaned everyday because of hair and improper solid waste disposal. Beside the screens, there was a small pile of hair, large particles of dirt, and a Cream-O’s wrapper.

 

The next treatment step occurs in the grit chamber. Here, the wastewater is contained and held for a short while for remaining particles to settle down to the bottom of the chamber. This settling process is repeated another two times later on in the STP.

As mentioned earlier, your waste composition changes daily. That means the treatment will adjust based on the daily composition of the sewage. The effluent from the grit chamber is directed to the equalization tank. The equalization tank serves two purposes: homogenization and dissolved oxygen addition. Homogenization ensures that composition throughout the effluent is uniform so that the STP does not have to constantly adjust to the different compositions. Because the composition of the effluent is now uniform throughout the whole effluent volume, its composition is also determined here. A sample from the equalization tank is sent to a nearby lab,  and the results are received within the day. The treatment process here also includes the addition of small amounts of dissolved oxygen so that the decomposition of organic matter can begin slowly without a huge shock to the system in the aeration tank.

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Simplified sketch of the oxygen diffusers located at the bottom of the aeration tank

Before the effluent reaches the aeration tank, it undergoes settling again in the primary sedimentation tanks. After settling, the actual chemical treatment of the waste begins. Depending on the composition of the waste, 0.5 to 1 ppm dissolved oxygen is released into the waste through diffusers at the bottom of the tank. Simply put, oxygen oxidizes the organic matter and breaks it down into simpler compounds. Unfortunately, the diffusers aren’t noticeable from the surface because the water is dark gray. The aeration tank also contains activated sludge, which contains microorganisms, to aid the decomposition.

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Secondary sedimentation (the clean water passes through those V notches!)

After the decomposition of organic matter, the wastewater passes through the secondary sedimentation tank where settling is repeated. This tank contains rectangular clarifiers. Each clarifier has a V-notch weir at the top where the clean water passes through. A suction is located at the bottom of the clarifier for the sludge to be pumped to the sludge tank. The sludge is separated into two types: return activated sludge and waste activated sludge. Return activated sludge is recycled for the process while waste activated sludge is sent to a belt press for the removal of water. The dewatered waste activated sludge is used as fertilizer. Although the bottom of the tank still can’t be seen, the water now has a lighter gray color!

The final step is disinfection using sodium hypochlorite (NaOCl). Sodium hypochlorite is widely used in detergents and bleaching agents. Scientists haven’t determined the exact mechanism by which sodium hypochlorite kills germs, but it is speculated that sodium hypochlorite inhibits any enzyme promoting bacterial growth and damages the DNA of the germs. In aqueous solution, NaOCl exists as three forms with HOCl being one of them. Because of its electrical neutrality and small size, HOCl penetrates the bacteria and can attack from both the inside and outside of the cell, guaranteeing its effectivity as a disinfectant. NaOCl works effectively at a particular pH so Manila Water regularly checks pH throughout the whole process to maintain the wastewater pH at a range of 6.5-9. 2

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The final product! The green blobs are the bunches of water lily

The resulting water isn’t clear but it is light brown and Class C water (i.e., it’s suitable for aquaculture). All the treatment water is released into the river through a single big pipe.

Maybe, you’re wondering why not mimic Singapore and produce potable water from the wastewater? 3 Well, it’s very expensive, and, although MWC has received proposals, there are no final plans. If it does happen in the future, the environmental cost, which is used to fund wastewater treatment, included in your monthly water bill may increase.

Unless you really enjoy wastewater treatment, we can’t tell you to think of better solutions for wastewater treatment especially for those communities without access to it. But, just please don’t forget to dispose of all your plastic wrappers and trash properly! Imagine having to clean the screens and sewage lines several times because of so much trash. It could clog the system and more frequent cleaning is time-consuming!

If you’re interested in your own DIY wastewater treatment, you should check out composting toilets and recycled greywater. Keep in mind that the monthly cost may be more expensive than your monthly environmental cost. You would also have to check with your local government unit about the possibility of having your own system because they would have to ensure your neighbors are also safe from toxic chemicals. But, having your own wastewater system is a good option if you are not connected to any treatment plant and live in a more “isolated” area.

If you have any questions, comment below! Thank you Manila Water for accommodating BOX at Olandes STP!


** Just a cool thing to note: Remember Ondoy and the flood it brought? Since Olandes STP is located underground, it had to withstand that flood. MWC had to ensure no untreated/partially-treated wastewater leaked out and contaminated everything. Because the Olandes STP survived the flood, it actually won an award!


Sources:

[1] Wilson, M. (2013, March 1). Infographic: Everything you were afraid to ask about poop. Fast Company. Retrieved from: http://www.fastcodesign.com/1672008/infographic-everything-you-were-afraid-to-ask-about-poop

[2] Fukuzaki, S. (2006). Mechanisms of actions of sodium hypochlorite in cleaning and disinfection processes. Biocontrol Science, 11 (4), 147-157.

[3] Galbaraith, K. (2012, July 25). Taking the ick factor out of recycled water. The New York Times. Retrieved from : http://www.nytimes.com/2012/07/26/business/global/26iht-green26.html?_r=0

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