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Containerized sewage treatment

Domestic sewage generated from small communities tends to have a variable and often high organic load.  The conductivity of the waste water is a function of chemicals used at the facility and this too is variable. From our experience, removal of the waste water organic load is the most capital intensive.  Best possible management can only partially reduce organic discharge. Organic and nutrient load in the waste water is analogous to sugar in coffee.  It cannot be filtered out with conventional filtration.  Like sugar it is biodegradable and can be removed successfully in a carefully designed biological environment.  Domestic sewage usually has a COD concentration of < 1000 mg/l. The flow to the works will generally contain the following;

  • Organic and inorganic solids. The solids may be biodegradable or non-biodegradable (e.g. ear buds and unmentionables)
  • Nutrients, mainly in the form of nitrogen and phosphorus
  • Ammonia
  • Grit (mainly small amounts of sand) Long and aging sewer networks tend to collect more sand - broken joints and cracked pipes being the points of entry

Wastewater is pumped or flows by gravity to the treatment works.

Treatment procedure

Inlet works A pump sump arrangement is supplied with the treatment plant, including a solids handling sewage pump.  The pump sump is a cylindrical shaped tank with submersible pump support brackets and guides.  Inlet heights are site specific and accommodated accordingly.   Biological treatment Organic treatment of the wastewater takes place in a high rate activated sludge bioreactor.  The bioreactor is sized and aerated to remove COD, nitrogen and phosphorous.  The activated-sludge process is a biological method of wastewater treatment that is performed by a variable and mixed community of microorganisms in an aerobic aquatic environment. These microorganisms derive energy from carbonaceous organic matter in aerated wastewater for the production of new cells in a process known as synthesis, while simultaneously releasing energy through the conversion of this organic matter into compounds that contain lower energy, such as carbon dioxide and water, in a process called respiration. This consortium of microorganisms is known collectively as biomass.   Hydraulic flow and organic load to the treatment plant is variable.  It is expected that at times the biomass population will exceed the available process volume.  This process is monitored by regularly measuring the sludge volume index (SVI).  When the SVI reaches a predetermined maximum value, a portion of the aerated biomass is discharged to a drying filter which is integrated above the shipping container.   Dried biomass will be periodically removed and can be used in the landscaped areas of the facility as an organic supplement. Screening The Dragon Sock™ is a highly effective screening mechanism developed and used by HWT.  The Dragon Sock™ is positioned at the post-anoxic overflow to aeration zone.  Screenings are gently captured in the Sock and simultaneously cleaned in the aerobic bioreactor.     Treated Effluent quality The South African Department of Water Affairs standards are used a guideline (see Table 1 below).  Treatment   requirements vary from country to country.  It is generally accepted that the South African limits are of world class standard, well within WHO recommendations.