Most Philippine cities have drainage systems of some kind. There are no separate storm water and waste water (sewerage) systems. Wastewater from septic systems freely mixes with the stormwater. In practice this is not as bad as it sounds as the volume of storm water is large and may well flush out the drainage system and its contents into the streams and rivers. The installation and maintenance of drainage systems (along with roads and water systems), is a one of the major responsibility of local government.
The systems are not necessarily centralized. The idea is to get wastewater to a nearby ditch, stream or river. Most residences will have a septic tank or cesspool to treat toilet waste. This can provide some very basic level of treatment of human waste. The output of septic system (septage) is piped to the subdivision or barangay drainage system. For a description of how we built our Philippine septic system CLICK HERE.
Greywater (wastewater from sinks, showers and washing) is generally discharged untreated into the drainage system. Sometimes the drainage system is more or less underground and sometimes it is open concrete ditches. This can be smelly but cheaper and easier to keep clear than covered drainage systems. In many places the drainage system is under the sidewalks, the sidewalks consisting of concrete slabs covering the drainage channel. Sometimes these are broken, so walk carefully or plunge a few feet into the malodorous mess. There is no further treatment of the waste before it plunges into the nearest stream.
In squatter areas the the above scenario breaks down and there can be more obvious, surface pollution.
Iloilo has some big rivers with strong tidal flows so all that goes into the rivers seems to get promptly flushed away. My impression is that the situation is better here than in Cebu City where the streams used for drainage were really disgusting.
When you look at property, ask about and confirm the easy availability of a good drainage system. We have looked at property in Iloilo City which seemed so nice but actually had no access to a drainage system. Further, access to the nearest drainage would involve going through other properties. This may or may not be easy. Do NOT expect anyone to inform you of such problems when you look at property.