Introduction and process
The wastewater prior to the AES system undergoes primary treatment in a septic tank. The volume of the septic tank is required to be at least 3 x Daily Design Flow. This allows for the accumulation of settled solids and floating scum, while maintaining a minimum hydraulic retention time of at least 24 hours for at least three to five years.
In addition to the settlement of denser organic solids, the septic tank incepts inorganic materials such as hand/baby wipes, pads, tampons, condoms, rags and other problematic materials that are disposed of in the wastewater system.
The primary treated septic tank effluent has the following quality characteristics1:
- Moderate to High suspended solids (65 to 180 mg/L)
- High total and soluble Biochemical Oxygen Demand (BOD5) (120-240 mg/L)
Nitrogen is present in the septic tank effluent as ammonia or Total Kjeldahl Nitrogen (TKN). The total nitrogen concentration of nitrogen in wastewater typically ranges from 50 mg / L to 75 mg / L. The primary treated effluent does not contain any nitrogen in the nitrate form.
The height difference between high and low vents and the hydraulic flows pulsing through the AES pipes constantly turns over the air within the system, thereby maintaining an aerobic environment. This is achieved without the need for blowers or pumps. Based on the surface area within the pipe and oxygen transfer rates, the optimum treatment rate specified by the manufacturer for the treatment of wastewater is 38 L per linear meter of AES pipe / day.
The organic solids that remain suspended in the septic tank effluent are retained within AES pipes. These solids are biologically degraded to soluble BOD5 in an aerobic environment. The effluent and soluble BOD5 will gravitate down through the AES pipe or be dawn out by the capillary action of the surround sand. This effluent migrates through the AES geo-textile and bio-accelerator fabric. Figure 1, next page shows the AES pipe cross-section and these layers.
Soluble BOD5 not consumed by the micro-organisms populating the AES pipe and fabric matrix, will move through to the next layer. The ammonia / TKN nitrogen is biologically oxidised to nitrate, which is soluble and will migrate through to the next layer as well.
The second treatment process layer is a 300 mm deep layer of clean course sand. This layer is effectively a slow sand filter that polishes the effluent by removing the last of the fine suspended solids and BOD5 from the effluent. This is a well proven treatment technique that produces a very high quality effluent with very low suspended solids and BOD5.
As the water moves through this sand layer the dissolved oxygen is used by the biomass as it consumes the BOD5. Once all the dissolved oxygen has been consumed (anoxic), biological de-nitrification will commence. This de-nitrification will continue until the last of the BOD5 is consumed. The effluent from this sand layer has the following characteristics:
- Very low in suspended solids
- Very low BOD5
- Very low ammonia
- Nitrogen present as nitrate
AES servicing this 15 bed building (Trefoil House) was brought into service at the end of November 2012. The building was then used throughout December 2012 but was vacant for all of January 2013. This effluent has been held in storage for just over 4 weeks.