The most important of natural purification processes are those biochemical changes that decompose organic compounds.


Anthropogenous interactions with water

Anthropogenous interactions with water occur in three main fields:
 household area;
 husbandry / agriculture industry
 industry.

The main sources of water pollution are:
 building activities (erosion, fertilizers, etc.);
 diary farming, agricultural activities (fertilizers, herbicides, pesticides, erosion, animal wastes concentration);
 automobile industry expansion (erosion, deicing chemicals, acid rain, other products connected with cars);
 power generation (fossil fuels – acid rain, spilling oil during transportation, warming up the wastewater, construction of the dams, spilling the radioactive materials into waterways);
 landfills (contamination of groundwater, illegal waste burial sites);
 household products (paint removers, bathroom cleansers, improper disposal of containers, washing powders);
 not existing canalization system in villages;

There are also problems as a result of a bad agricultural management in the past years:
 Extensive production of the cattle and pigs caused the total damage of soils by animal wastes in heavy concentrations;
 Overdose of chemical fertilizers;
 Agricultural lands were placed very closely to the pollution sources.


WASTE WATER TREATMENT

There are 4 main categories of wastewater and thus wastewater sewage systems:
1. Domestic sewage – wastewater from households, commercials, services, schools, administrative, etc.
2. Industrial wastewater – wastewater produced in various industrial branches, in agro-industries and forestry.
3. Municipal wastewater – sewage system, in which we cannot distinguish domestic wastewater from industrial WW; they are not in separated sewage systems
4. Rainwater – water polluted into some extent. Rainwater carries dust particles from the atmosphere to the ground, it dissolves also a little oxygen, nitrogen, and carbon dioxide as it falls through the atmosphere. During electrical storms, lighting causes nitrogen, oxygen, and water vapor to form nitric acid. Traces of this are also found in rainwater. Rainwater also dissolves and washes various chemical compounds from roads, streets and fertilizers from fields.

There are 2 types of wastewater treatment:
1. Pre-treatment – treatment of wastewater before its discharging onto the sewer (this is used for strongly polluted industrial wastewater)
2. Full wastewater treatment – treatment of wastewater before its discharging into the recipient (flow, lake, sea)

Biological wastewater treatment
 Final aim is to remove organic matter from wastewater
 Main components or actors in biological WWT are substrate – an organic matter; and a biomass – microorganisms.
 Substrate - organic material contained in domestic WW, in agro-industrial WW, in food industry WW (all these WW are compatible with biological WWT) and can be decomposed by microorganisms. Substrate contains 3 main element that are crucial for bacteria life: N- nitrogen, P- phosphorus, and K-potassium.  The organic matter biodegradation can be either aerobic or anaerobic. Aerobic oxidation occurs in the presence of dissolved oxygen. A measure of the amount of oxygen needed for this degradation is the biochemical oxygen demand (BOD). The higher the quantity of degradable organic wastes the higher the BOD.
 Flowing streams (thus wall aerated) are able to regenerate themselves. In rapids, the moving water dissolves oxygen. Lakes with little or no flow can remain dead for decades.
 Eutrophisation – a problem that can arise in still waters, which is fed by nutrients (nitrates and phosphates). This may serve as nutrient for the growth of algae. These algal blooms are stimulated by the runoff of agricultural fertilizers and by phosphates in detergents.