Ecology of Wastewater

In a closed ecosystem, there are no such thing as wastes. Just about everything is eventually recycled. For example, as animals breath out carbon dioxide, plants thrive on the carbon dioxide and release oxygen. So too do plants thrive on natural fertilizer from the droppings of animals and in return provide food to others in the animal kingdom. Nutrients, water, and carbon are constantly being recycled in the ecosystem.

The treatment of wastewater from humans borrows upon nature. The most popular wastewater treatment methods are simply amplifications of what occurs naturally. Microorganisms which would normally break down the waste products are cultured, harnessed, and pampered with "food" and oxygen so they break down the wastewater more quickly than usual. In general, obnoxious compounds are broken down and removed and the water (with some impurities) is returned back to the ecosystem. This type of treatment is called secondary treatment.

The City and County of Honolulu has six plants which use this natural method of treating wastewater. They are Kailua, Wahiawa, Kahuku, Waianae, Waimanalo, and Paalaa Kai Wastewater Treatment Plants. The treated wastewater is either injected into the ground, discharged into a lake, or discharged into the ocean. These discharges return the water and left-over impurities for recycling back into our ecosystem.

The City is also planning for possible direct reuse of treated wastewater. Direct reuse means using the wastewater for irrigation or industrial processes. Before wastewater can be applied for direct reuse, in most cases it has to be further polished after secondary treatment and disinfected.

Another type of treatment is called primary treatment. Primary treatment involves removing only wastes which settle out of the wastewater. Discharge of primary treated wastewater is only allowed into the deep ocean. In the deep ocean, the dilution, available oxygen, and distance from land allows the natural treatment process to occur away from land. However, land-locked mainland cities are required to have secondary treatment since their wastewater is discharged into rivers, streams and lakes.

The City and County of Honolulu has two plants which use primary treatment, namely the Sand Island and Honouliuli Wastewater Treatment Plants. These two plants treat most of the Wastewater (more than 85%) for the City.

Primary treatment discharges require extensive monitoring of the ocean to determine effects of the discharge. The City currently monitors bacteria levels, micromollusk (tiny seashell organisms) populations, over 64 chemicals in the flesh of fish caught in the area, abnormal growths in the livers of fish caught in the area, nutrient levels (nitrogen, phosphorus), chlorophyl levels (which is related to algae growth), turbidity, temperature, and properties of the sand and sediment from the sea floor. The data from these tests help to determine how the ocean environment is changing and whether the wastewater discharge is having any effects.

The solids removed during the wastewater treatment process can also be treated for recycling as fertilizer or soil enhancers. In the treated stage, the solids are called "biosolids" and provide nutrient values competitive with chemical or animal-derived fertilizers.

Currently, the City disposes of the wastewater solids in the landfills. However, the City is in working to process the solids into a useful product. Recycling of the solids will help to conserve dwindling landfill space.