What happens to the stuff you flush down the toilet?

Dealing with the “icky” stuff can be a bit tricky. One option is to simply dump everything directly into the ocean. However, that would be a very, very (stress on the “very”) bad thing to do. Good thing that that’s not the case here in El Nido. El Nido Resorts operates sewage treatment plants (STPs) on its island resorts that treat all the wastewater coming from the cottages, Clubhouse complex, and staff quarters.

Wastewater consists of raw sewage (also known as “black water”) coming form the toilets and kitchen drains, and gray water from the bath and sinks. Laundry water is treated separately, as it contains chemicals that don’t agree with the STP.

How we do it

The key processes in wastewater treatment here are 1) the removal of oil and grease; 2) anaerobic digestion using an Upflow Anaerobic Sludge Blanket; 3) aerobic digestion using the Sequencing Batch Reactor; and 4) tertiary treatment. In the end, the solids in the raw sewage are converted to water, gas, and organic ash, while the product water is used for flushing toilets and watering the plants. The organic ash is used as a low-grade soil conditioner.

Fats, oils, and grease don’t play well with the STP, so grease traps are installed in the plumbing to remove them before the wastewater even gets to the lift station. After passing through the grease traps, the wastewater gets to the lift station and is then pumped up to the STP.

The wastewater’s first stop in the STP is the pre-aeration tank, where it passes through a bar screen. This is where large solids such as pieces of cloth, plastics, and other debris are caught and manually removed for proper disposal. From the pre-aeration tank, the wastewater goes to the equalization tank where it is homogenized. This is also where excess grease that escaped the grease trap is manually skimmed off the top. The equalization tank is aerated through an air line connected to the air blower with diffusers laid out at the bottom of the tank.

After the equalization tank is the Upflow Anaerobic Sludge Blanket (UASB). The wastewater is pumped out near the bottom of the tank and is equally distributed over its total area. Above the wastewater is a carefully maintained sludge blanket through which the wastewater is forced to percolate upwards. The anaerobic bacteria present in the sludge digests the solid waste, resulting in clearer water. The dead bacteria sink to the bottom of the tank, also resulting in clearer water. A specific upflow velocity should be maintained to keep the sludge blanket in suspension.

Next after the UASB is aerobic digestion through the Sequencing Batch Reactor (SBR). Wastewater inside the SBR undergoes five discrete processes: Fill, Mix, Aerate, Settle, and Decant. First, the reactor is filled with partially treated wastewater that went through trickling filtration. The Fill phase takes about an hour. Second, the wastewater mixes with the aerobic bacteria already present in the reactor. Third, the mixture is then aerated to support the aerobic bacteria as they digest the organic matter. The reaction phase takes about two hours. Once aeration and reaction is completed, the flowing air is turned off and the mixture is allowed to settle, separating the clear water from the aerobic bacteria. Settling time is about one hour. And last, the clear water above the settled bacteria is drawn off by floating decant pumps for final discharge. Decantation takes about an hour or more.

After SBR, the only things left to do are to chlorinate the water and to filter it through sand. The chlorine kills any remaining pathogenic bacteria, while the mechanical sand filtration removes the last bits of the suspended solids. The resulting clean product water is used for flushing the toilets, supplying the fire hydrants, and watering the gardens, while the sewage sludge is sundried then used as garden fertilizer.