Where Does the Water Go?


PANIC! That was my first reaction. A gray liquid bubbling up from my bathroom floor drain was threatening to turn my apartment into a smelly swamp. Urgently I called the plumber for help. As I waited despondently, with my mouth nervously dry and water slowly soaking into my socks, I wondered, ‘Where did all that water come from?’

While the plumber busily coaxed the blockage from the drain, he explained: “The average city dweller uses 200 to 400 liters [50 to 100 gallons] of water a day. For every man, woman, and child, about 100,000 liters [25,000 gallons] a year goes down the drain.” I asked: “How could I possibly use that much water? I certainly don’t drink it!” “No,” he replied, “but each day you take a shower or a bath, flush the toilet, and perhaps use a washing machine or a dishwasher. In these and other ways, the modern life-style causes us to use twice as much water as our grandparents did.” The question then sprang to my mind, ‘Where does all that water go?’

I discovered that the water we throw away each day is treated very differently depending on the country or even the city where we live. In some countries this is now a life-and-death issue. (See the boxes on page 27.) Come with me on a tour of my local wastewater treatment plant and discover for yourself where the water goes and why it pays to think carefully before you put things down the drain or toilet, no matter where you live.

Into the Treatment Works

I know you are thinking that a wastewater treatment plant doesn’t sound like an attractive place to visit. I agree. Yet, most of us depend on such a plant to keep our city from drowning in its own waste​—and we all play a part in helping these plants to function properly. Our destination is the primary treatment plant at Malabar, just south of the famous Sydney Harbor. How does the water from my bathroom get to the plant?

When I flush the toilet, empty the sink, or take a shower, the water travels toward the wastewater treatment plant. After a journey of 30 miles [50 km], this water joins the 130 million gallons [480 million L] a day gushing into the treatment plant.

Explaining why this treatment plant is not unpleasant to the eye and nose, Ross, the plant’s community liaison officer, told me: “The majority of the plant is buried below ground. This allows us to trap the gases and funnel them to the  air scrubbers (a row of giant pot-shaped chimneys), which neutralize the offensive odors. The cleansed air is then released into the atmosphere. Although the plant is surrounded by thousands of houses, I only get about ten calls a year complaining about odor problems.” Inevitably, the location Ross is taking us to next is the source of those “odor problems.”

What Is Wastewater?

As we descend deeper into the plant, our guide tells us: “Wastewater is 99.9 percent water plus human waste, chemicals, and various other bits and pieces. The wastewater collected from houses and industries over a 55,000-hectare [130,000 acre] area, along 20,000 kilometers [12,000 miles] of pipes, enters the plant two meters  [6 feet] below sea level. Here it passes through a series of screens that sift out rags, rocks, paper, and plastic. Next, in the grit chambers, organic matter is suspended in the water by air bubbles, and the heavier grit settles to the bottom. All this inorganic waste is collected and sent to a landfill. The remaining wastewater is pumped up 15 meters [50 feet] to the sedimentation tanks.”

These tanks cover an area about the size of a soccer field, and it is here that you realize how much the neighbors would complain if the air-purifying system were not so effective. As the water slowly flows through the tanks, oil and grease float to the surface and are scraped off. The fine solids, called sludge, settle to the bottom, and great mechanical blades scrape the sediment to where it is pumped away for further treatment.

The processed wastewater flows out to sea through an underground outfall tunnel two miles [3 km] long. There the water rises to the ocean floor and diffuses into the sea, 200 to 300 feet [60-80 m] below the waves. Strong coastal currents disperse the wastewater, and the natural disinfecting quality of salt water finishes the treatment process. The sludge remaining at the treatment plant is pumped into large tanks called anaerobic digesters, where microorganisms break down the organic matter into methane gas and a more stable sludge.

From Sludge to Soil

With a sigh of relief, I follow Ross back up to the fresh air, and we climb to the top of one of the airtight sludge tanks. He continues: “The methane produced by the microorganisms is used to power electric generators and provides over 60 percent of the power for plant operation. The stabilized sludge is disinfected, and lime is added, transforming it into a useful material, rich in plant nutrients called biosolids. The Malabar Sewage Treatment Plant alone produces 40,000 tons of biosolids yearly. Ten years ago the untreated sludge was incinerated or dumped into the ocean; now this resource is put to better use.”

Ross hands me a brochure that explains: “Forests of [New South Wales] show increased growth responses of between 20 and 35 per cent after biosolids have been applied.” It also states that growing ‘wheat in soils applied with biosolids has produced increased yields of up to 70 percent.’ I notice that composted biosolids are now safe enough for me to use for fertilizing the flowers in my garden.

Out of Sight, Out of Mind?

At the end of the tour, our guide reminds me that pouring paint, pesticides, medicines, or oil down the drain may cause the microorganisms at the treatment plant to die and could thus disrupt the recycling process. He stresses that ‘oils and fats slowly clog the arteries of our plumbing systems in the same way that they clog our own arteries and that disposable diapers, cloth, and plastic flushed down the toilet don’t go away. Instead, they block the pipes.’ As I have learned, rubbish may be flushed out of sight, but when the water backs up the drain, it soon comes back to mind. So the next time you take a shower, flush the toilet, or empty a sink, think about where the water goes.

[Box/Picture on page 25]

From Wastewater to Drinking Water

The several million inhabitants of Orange County​—a low rainfall area of California, in the United States—​benefit from an innovative solution to the wastewater problem. Instead of millions of gallons of wastewater being dumped directly into the ocean each day, the majority is returned to the water supply. For many years this feat has been achieved by a wastewater treatment plant. After primary treatment, the wastewater undergoes secondary and tertiary treatment. This involves purifying the water so that it is as clean as conventional drinking water. It is then mixed with deep-well water and goes into the groundwater basin. There it replenishes the basin and also prevents salt water from seeping in and ruining the groundwater reservoir. Up to 75 percent of the district’s total water needs are drawn from this underground supply.

[Box on page 27]

Five Ways to Be Waterwise

▪ Replace leaky washers​—a dripping tap can waste 2,000 gallons [7,000 L] a year.

▪ Check that your toilet does not leak​—it can waste 4,000 gallons [16,000 L] a year.

▪ Install a water-efficient shower head. A standard shower head delivers 4.5 gallons [18 L] a minute; a low-flow shower head provides 2.5 gallons [9 L] a minute. A family of four will save up to 20,000 gallons [80,000 L] a year.

▪ If you have a dual-flush toilet, use the half-flush button when appropriate​—this saves more than 9,000 gallons [36,000 L] a year for a four-person family.

▪ Install an aerator on your taps​—they are relatively inexpensive and reduce water flow by half without reducing usefulness.

[Box on page 27]

World Wastewater Crisis

“More than 1.2 billion people still lack access to clean drinking water while 2.9 billion lack access to adequate sanitation facilities, resulting in an annual death rate of 5 million persons, largely children, from water-borne diseases.”​—The Second World Water Forum held at The Hague in the Netherlands.

[Diagram/Pictures on page 26]

(For fully formatted text, see publication)

The Wastewater Treatment Process at Malabar (Simplified view)

1. Wastewater enters the plant

2. Screening

3. Grit chambers

4. To landfill site

5. Sedimentation tanks

6. To the ocean

7. Anaerobic digesters

8. Electric generators

9. Biosolid storage tank


Anaerobic digesting tanks turn sludge into useful fertilizer and methane gas

Methane gas is burned to generate electricity