Household Utilities: Sewage: Black Water Options.

In Queensland at "June 1996, 185 sewage treatment plants, 12 water supply treatment plants and 136 industry treatment plants were licensed to discharge waste water into estuaries and marine waters. In total, sewage treatment plants discharged an estimated 25,280 kg/day of nitrogen and 7220 kg/day of phosphorus" (QGEPA, 1999).

Black water is all sewage waste. The average person produces 0.14 - 0.28 kg of faecal material per day and 1200 ml of urine. Residential houses accessing reticulated water use about 1/3 of the daily demand of potable domestic water for flushing the toilet, or over 21,000 litres per year per household member.

Blackwater Options

1. Reticulated sewage system, modern dual flush toilets can use as little as 3 litres for half flush and 6 litres for full flush. The average person in Sydney flushes 23% of their drinking water down the sewage. 23% x 180L = 41.4 litres per day per person, or over 15000 litres per year per person.

2. Septic tanks systems have two components, a septic tank to provide partial treatment of the raw waste, and a disposal field. Passive anaerobic pre-treatment will remove 40-60% BOD5, 50-70% suspended solids, 10-30% nitrogen and 30% phosphorus. The effluent discharging from the tank then percolates through the soil, being filtered as it does. However a study by Geary in1998 showed that 40% of on-site septic systems in NSW are failing. By passing all household water through the same system would result in greywater being contaminated with sewage.

3. Dry Composting toilets usually need to be designed into the building from the start. They consist of a large chamber below the toilet pedestal so that waste will fall without flushing. The unit then evaporates or drains most of the moisture from the urine, and converts all the remaining waste into compost that can be used in the garden. In some cases composting toilets can also accept kitchen waste and paper. Heat from aerobic decomposition destroys pathogens, decomposes organic waste and evaporates the bulk of the water content. Nature-Loo dry composting toilet has been selected for the Newton House.

4. Low volume flush composting toilets use a combination of compressed air or a partial vacuum and a low volume of water.

5. Chemical toilets use a flushing liquid in a closed loop system to clean the toilet and sterilise the sewage waste.

Final Effluent contents

Pollutant	Quantity
BOD5	< 20 mg/L
Suspended Solids	< 30 mg/L
Faecal coliforms	<10 per 100 ml
Free residual chlorine	< 0.5 mg/L

Health Issues

Pathogens such as viruses, worms (especially eggs), amoebas, bacteria and protozoa are carried in blackwater and can be transferred to the soil or groundwater by ineffective treatment systems, including septic tanks. What kills pathogens is a combination of time and elevated temperatures - most pathogens are either destroyed or rendered harmless if kept at 50 degrees C for one day. However, Roundworm eggs can survive in the soil for several years. Therefore, composting toilets need either long retention times and/or are able to maintain elevated temperatures. Note that elevated temperatures without supplemental heating may be difficult to achieve.

Links

http://www.nature-loo.com.au
http://www.goldcoast.qld.gov.au/t_std2.asp?PID=295