Recycling wastewater and its solids

Water draining

If you live downstream of any town, some of the water you drink was once used to flush someone’s toilet. And so it’s past time to outgrow squeamishness about wastewater. It simply goes from our homes to a sewer to a wastewater treatment plant, where it’s cleaned up and returned to a river or lake.

And the solids the plant removes from the sewage? We also need to stop regarding them as a waste and recognize them as a resource.

Think of wastewater as contaminated water. It contains all kinds of bacteria, chemicals, and other toxic substances. Wastewater treatment cleans it up enough that it can be safely returned to nature. From there, some of it will flow into a water treatment plant to make it usable again.

In the home, wastewater comes from the toilet, bathroom and kitchen sinks, bathtub, and washing machine. From all those places, it flows through the same kind of pipe, into the sewer, and on to the wastewater treatment plant.

How we can carelessly create problems for wastewater treatment

It’s easy to flush or pull the drain plug and not consider what happens next. Certain things never belong in the sewer, such as

Fat, oil and grease from the kitchen
Food waste
Pills or medicine of any kind
Paint
Any wipes besides toilet paper—even if the package says flushable.
Feminine hygiene products
Motor oil down storm drains

Solids, especially large solids, can clog sewer pipes. So can fats and oils. Clogged sewer pipes can cause sewage to back up into someone’s house—maybe yours. And the water treatment process cannot remove dissolved medicines or other chemicals.

The wastewater treatment process

Sewer pipe in Brighton, England opening into an overflow weir.

Some municipalities have separate storm sewers and sanitary sewers. Others combine the two in one system. Each has advantages and disadvantages.

Wastewater stinks. So the first task at the wastewater treatment plant is to neutralize the odors with chemicals.

It is also full of what people flush or dump in sinks that they shouldn’t. If the municipality has a combined sewer, the sewage will also contain leaves, plastic bags, and whatever else rainwater flushes down the storm drains.

All this trash must be screened out before the water treatment can begin. A separate process removes sand, grit, and other inorganic solids. Then, all this material usually goes to the local landfill. Some wastewater treatment plants incinerate it to provide some of the power they need to operate.

Next, the wastewater goes to settling tanks for primary treatment. Remaining solids sink. Remaining oils float. Skimmers remove the oil and scum. Scrapers at the bottom of the tank remove the solids, now called sludge.

Secondary treatment takes place in another tank. It uses activated sludge, sludge seeded with various kinds of microorganisms. The process also involves pumping air into the wastewater. In that environment, the microorganisms feast on the dissolved organic matter. They also eventually clump together to form floc, which settles as more sludge.

Some treatment plants provide tertiary treatment, processes similar to how water treatment plants purify drinking water. Tertiary treatment removes nitrogen, phosphorous, and other nutrients that can promote the growth of algae in the lake or river that receives the plant’s effluent. It produces its own sludge.

Before the effluent from the treatment process can return to the river or lake, it is disinfected to kill any remaining pathogens. After about half an hour in a tank with chlorine and sodium hypochlorate, it is ready for discharge.

Treatment of sludge

Sewage treatment plant

Sewage sludge refers to the solids that settle out in the wastewater treatment process. It combines the solids that settle out in primary, secondary, and tertiary treatment, but it is still a liquid.

Raw sewage sludge, with 1-4% solids, contains pathogens. It is capable of further decomposition and is thus unstable.

Treatment of sludge entails a combination of several common methods.

Thickening concentrates the sludge to a solid content of 5 or 6%.
Dewatering uses various processes to increase it to 15-30%. Air drying the sludge on sand beds removes pathogens. Using centrifuges or filters removes some nutrients.
Anaerobic digestion takes place in the absence of air. Microorganisms that live in airless conditions feast on the sludge for anywhere from 15 to 60 days. This process further increases solid content and reduces odors, volatile solids, and pathogens. It preserves nutrients and produces methane. The treatment plant usually uses the methane to produce power to run the equipment.
Aerobic digestion uses microorganisms that breath air. It accomplishes the same purposes as anaerobic digestion with some loss of nitrogen. Instead of methane, it produces carbon dioxide.
Alkaline stabilization adds enough lime or other material to increase the pH of the sludge to 12 for 2 hours and make it stay above 11.5 for another 22 hours. This process further reduces pathogens and volatile solids. It causes loss of ammonia and may convert phosphorous to forms that plants can’t use.
Composting mixes sludge dewatered to about 20% solid content with sawdust or other high-carbon organic material. It remains for several days at temperatures of at least 131°F. Composting reduces the volume of the sludge and decreases odors, pathogens, and volatile solids. It therefore stabilizes the organic matter. Compost actually has less plant nutrient value than fertilizer that can be manufactured after completion of some of the other processes.

What to do with biosolids

This is what sludge land appication looks like when it is finished: a nearly even distribution of the sludge. Now, it has to be covered with soil.

After treatment, the raw sewage sludge has become biosolids. Biosolids can be viewed as a waste that requires disposal. In that case, it either goes to a landfill or an incinerator. Both of these methods have economic and environmental advantages and disadvantages.

Landfilling is now the cheapest and simplest option for dealing with biosolids, but as landfill space gets filled up, the cost to dump anything will increase. In the landfill, biosolids undergo anaerobic digestion, which produces methane. And methane is a potent greenhouse gas. On the other hand, heavy metals and other toxins become highly concentrated in the ash that remains after incineration.

Organic fertilizer

More creatively, biosolids can be viewed as a resource. They can safely be applied directly to farmland. Farmers who use biosolids do not need to buy inorganic fertilizers. What’s more, biosolids do not cause as much runoff or loss of nutrients by leaching as the same amount of commercial fertilizer.

Biosolids can also become raw material for making useful products. Subjected to processes such as pelletizing, pasteurizing, or composting, they become organic fertilizers. The city of Milwaukee has long packaged and sold fertilizer under the brand name Milorganite.

If you want to find organic fertilizer for your lawn or garden, your garden supply store probably has Milorganite. Since that is a registered trademark, biosolids from any other municipality must be sold under a different name.

Bricks with biosolids

Most of the biosolids from wastewater treatment plants eventually become fertilizer. About a third of it goes to landfills. Australian researchers have found another use. They have made bricks with between 10 and 25% biosolid content.

These bricks require less clay and therefore less clay mining. They meet all safety standards for bricks. While somewhat less sturdy than standard bricks, they are more porous. That is, they trap more gas bubbles inside, making lighter bricks that insulate better. What’s more, firing bricks with biosolids requires less energy. Those with 25% biosolids use only about half the energy needed to make regular bricks.

Any way we can convert waste products to resources helps the environment That it uses less space and energy in landfills only begins the advantages.