Municipal waste water treatment.

Municipal waste waters are generated in households and in public institutions. This type of waste water contains high level of nutritive and organic matter.
Treatment consists of the following phases: pre-cleaning (mechanical phase), primary cleaning, secondary cleaning (biological phase), tertiary cleaning, disinfection, sludge treatment.

Mechanical process is designed for removal of bigger particles (branches, rocks, glass, bottles, and rags) by means of rakes and sieves. Sand, gravel, egg shells, cigarette filters, seeds and grease are removed through grit and grease chambers. There are several types of grit chambers: gravity, ventilation, vortex grit chambers.

In the process of primary cleaning treatment sedimentary particles and a part of organic matter are removed. Lately, primary sludge settling tanks have been replaced with fine sieves in smaller treatment plants.

The biological cleaning treatment includes secondary and tertiary cleaning. Biochemical processes by means of microorganisms remove pollution. Secondary treatment is carried out under aerobic conditions by means of air injection through membrane diffusers. Organic matter is removed and nitrogen in form of ammonia is transformed into nitrites.
Tertiary treatment is carried out under anoxic conditions where microorganisms consume organically bound oxygen. Nitrogen and phosphorus are removed through this method.

Various technologies are used for water treatment:

  • Technologies imitating natural systems (lagoons, irrigation systems of fields); simple and cheap for maintenance, suitable for smaller settlements;
  • Classic continuous process of active sludge: tested, moderate investment and operational costs, great possibilities for optimisation, it needs much space;
  • SBR process of active sludge: smaller place for installation, lower investment costs, more complex operation, problems with floating sludge;
  • MBR (membrane bioreactor): smaller place for installation, high efficiency of cleaning, high operational costs;
  • MMBR (biological reactor with fitted biomass): lower investment costs, lower operational costs, more complex operation.

The sludge occurring in the process of water treatment is treated through various processes:

  • Concentration (gravity, mechanically ? centrifuges, belt thickeners);
  • Stabilisation (aerobic, anaerobic);
  • Dehydration (centrifuges, filter presses);
  • Drying process.

During the operation of treatment plants much energy can be saved if the process is optimised and power consumption minimised:

  • Optimisation of sludge flow and inflow structures of sludge settling tank;
  • Anaerobic decay, formation of bio gases and electrical power (suitable for larger treatment plants).