A package sewage treatment plant is a compact sewage treatment system suitable for treating the waste generated by populations of up to 5000 people.
There are two types of package sewage treatment plants:
Type 1 - Traditional 3 stage process.
A "Type 1" plant has a primary settlement chamber for solids to settle for removal by tanker later, a biozone stage for aerobic treatment of liquid and a final settlement chamber for fine suspended solids. The primary settlement chamber is simply a holding compartment where sewage entering the tank becomes still thus allowing settlement of solids by gravity. The biozone is the where the primary treated liquid is treated by aerobic bacteria that occur naturally in the sewage. The digestion process is accelerated in a sewage treatment plant through oxygenation. Bacteria are given a media on which they can colonize. This media allows bacteria to feed and breathe simultaneously. The media is fixed, rotating or floating. After the biozone the treated liquid enters a final settlement chamber which as the name suggests, simply allows final settlement of solids in suspension by gravity before the treated water leaves the plant.
Type 2 - Extended aeration process.
A "Type 2" plant introduces oxygen to the wastewater promoting growth of aerobic bacteria, which destroys organic compounds in the sewage. Incoming sewage enters the aeration chamber where air surging up from a draft tube oxidizes the sewage. This allows an aerobic bacterial colony to establish and digest the organic matter. After an appropriate time period in the aeration chamber the liquid flows into a clarification chamber. The calm conditions in this chamber allow settlement of activated sludge and small organic particles suspended in the liquid. Untreated particles are returned to the aeration chamber for final treatment leaving only purified clean effluent discharging from the system. By continuously recycling sewage via gravity and air lift, almost total digestion of solids is achieved. The advantage of this type of sewage treatment plant is the long (typically 3-5 year) desludge intervals."