Options in Biological Treatment

There are a variety of different technologies that can perform biological wastewater treatment. As technology advances, these systems are becoming increasingly convenient, more efficient, and smaller in size.

Most common available options in the biological treatment processes of domestic sewage options are

  • Trickling Filter (TF)
  • Activated Sludge Process (ASP)
  • Submerged Aerated Filter (SAF)
  • Moving Bed Biological Reactor (MBBR), and
  • Membrane Biological Reactor (MBR)

The first two methods are conventional treatment processes, the next four methods are low cost methods and the last two methods are emerging technologies.

Trickling Filters

Trickling filters are intended to treat particularly strong or variable organic loads. They are typically circular filters filled with open stone or synthetic filter media to which wastewater is applied at a relatively high rate. The design of the filters allows high hydraulic loading and a high flow-through of air. On larger installations, air is forced through the media using blowers. The resultant liquor is usually within the normal range for conventional treatment processes.

Activated Sludge Process

The activated sludge process (ASP) is an aerobic biological wastewater treatment process that uses microorganisms, including bacteria, fungi, and protozoa, to speed up decomposition of organic matter requiring oxygen for treatment. In this process, microorganisms are thoroughly mixed with organics under conditions that stimulate their growth and waste materials are removed. Activated sludge plants use a variety of mechanisms and processes to use dissolved oxygen to promote the growth of biological floc that substantially removes organic material. A portion of the settled sludge is returned to the aeration tank (and hence is called return sludge) to maintain an optimum concentration of acclimated microorganisms in the aeration tank to break down the organics. It also traps particulate material and can, under ideal conditions, convert ammonia to nitrite and ultimately to nitrogen gas.

Submerged Aerated Filter

Submerged Aerated Filter (SAF) is a well proven technology for wastewater treatment. Technology is seen as the simplest and most cost effective method of commercial and residential sewage treatment / waste water treatment, particularly for small to medium sized treatment plants where available land is limited, and where full time operational manning would be uneconomical. A well-built Submerged Aerated Filter plant has no moving parts within its main process zones, any serviceable items will be positioned to access easily without disrupting the ongoing sewage treatment particularly when using random filter media. Submerged Aerated Filter (SAF) technology is a process used to reduce the organic loading of residential and commercial sewage / waste water, and in doing so will reduce the Biological Oxygen Demand (BOD) and a significant quantity of Suspended Solids (SS) which if otherwise untreated would contaminate river and sea outfalls, in other words it is used to substantially improve effluent discharge quality.

As with traditional effluent treatment, Submerged Aerated Filter technology uses three stages of dealing with commercial and residential sewage / waste water:

  • Primary Settlement Where larger solids settle into the bottom of the primary tank and are removed periodically as sludge, and where other buoyant materials float upwards to be removed usually by a scraping/screening method.
  • Secondary Treatment Where larger solids settle into the bottom of the primary tank and are removed periodically as sludge, and where other buoyant materials float upwards to be removed usually by a scraping/screening method.
  • Final Settlement / Clarification Where remaining solids (Humus) are settled out of the biological treated effluent.

Moving Bed Biological Reactor

Moving Bed Biological Reactor (MBBR) involves the addition of inert media into existing activated sludge basins to provide active sites for biomass attachment. This conversion results in a strictly attached growth system.

Membrane Biological Reactors

Membrane Biological Reactors (MBR) includes a semi-permeable membrane barrier system either submerged or in conjunction with an activated sludge process. This technology guarantees removal of all suspended and some dissolved pollutants. The limitation of MBR systems is directly proportional to nutrient reduction efficiency of the activated sludge process. The cost of building and operating a MBR is usually higher than conventional wastewater treatment.