Drinking Water Treatment and Greenhouse Gases

After reading my recent post on making your own and water conservation, someone remarked to me that they'd never thought about how their drinking water is treated, let alone how it gets to their house. In the same way, most people don't give any further consideration to their wastewater once they've flushed to loo or pulled the plug! Since my background is in water and wastewater treatment, I thought it might be useful to give a brief outline of the processes involved.

Water Treatment

Treating water to drinking standards involves firstly pumping the "raw" untreated water from the local supply (dam or river for example) to the treatment plant. The solid material is then allowed to settle out (called sedimentation). Usually, this process is helped along by adding chemicals ("flocculants") to the water which aid in forming large particles ("flocs") which settle out of the water more rapidly. The water is then usually filtered to remove smaller particles which have remained in suspension. Traditionally this was done by gravity trickling the water through large tanks comprising layers of sand, charcoal and gravel. In more modern plants, membrane filtration is used, where the water is forced through membranes with very fine pores, leaving behind most contaminants, including most residual parasites, bacteria and parasites. The water is then commonly pH adjusted using chemicals (so that it doesn't corrode your copper pipes for example), and disinfected, usually using chlorine (chlorination). It is then usually then pumped to reservoirs, and gravity-fed or pumped to your home via an undergound pipe network (reticulation).

Wastewater (sewage) Treatment

Once the wastewater leaves your home, it flows through the underground pipes (sewerage system), usually by gravity, to localised collection stations, where it is then pumped (along with industrial effluent) to the local sewage treatment plant. Wastewater is generally 99.99% water.

The wastewater is screened to remove large objects, then it is allowed to settle (sedimentation, "primary treatment") like the water treatment process. Secondary treatment consists of using bacteria to break down the dissolved and remaining suspended solids in the water. The sludge from both the primary and secondary stages is pumped to another area for further treatment. The water from the secondary stage is then disinfected, usually with chemicals such as chlorine, to kill the remaining bacteria. In older systems, sunlight was used as the disinfectant by holding the water in large treatment ponds for a minimum period of time. The treated water is then commonly discharged into the local river or ocean outfall. (It's worth noting that most plants with ocean outfalls don't have to disinfect the wastewater).

The sludge is treated by heating it slightly and allowing bacteria to break down the biodegradable material into harmless solids, which is ideal as a soil conditioner. This process gives off methane, a greenhouse gas. In modern plants, this methane is harvested and used to produce electricity to power the other treatment processes. Note: solid kitchen waste such as food scraps, oil and vegetable peelings, add a huge extra load to the treatment process (therefore requiring extra energy) - don't put them down the sink and don't use an in-sink-erator if you have one!

So, where do greenhouse gases come into it? According to Gippsland Water, "The water industry (including wastewater treatment) accounts for 0.5% of greenhouse gas emissions in Australia. This impact is primarily due to methane and nitrous oxide emissions and electricity consumption during waste treatment processes, as well as energy consumption in the pumping of large volumes of water."

So as you can see, if you don't have your own water supply and/or wastewater treatment (e.g. a septic tank), conserving water also conserves energy. That includes harvesting and using rainwater, and reusing greywater.

You can also see from the above that wastewater reuse as drinking water not only conserves water, but energy as well. I know from personal experience that effluent being discharged from a treatment plant is often better quality than the raw water used for the drinking water supply, because of the stringent EPA license requirements. If you are going to spend all that energy and all those materials treating wastewater to such exacting environmental standards, doesn't it make more sense to take it one stage further in treatment and use it as drinking water? Trust me, having spent years testing raw water supplies (note that in country NSW, most effluent is discharged into rivers... which then becomes the raw drinking water supply for the next town down the river), I am not the slightest bit squeamish about drinking recycled water!

What about desalination? Current desalination (purifying seawater) processes are really energy hungry and produce a lot of waste. Check out this report by the Pacific Institute, about desalination in California. It states that "The potential benefits of ocean desalination are great, but the economic, cultural, and environmental costs of wide commercialization remain high. In many parts of the world, alternatives can provide the same freshwater benefits of ocean desalination at far lower economic and environmental costs. These alternatives include treating low-quality local water sources, encouraging regional water transfers, improving conservation and efficiency, accelerating wastewater recycling and reuse, and implementing smart land-use planning". That doesn't sound very sensible to me ;-)