Biological Wastewater Treatment System

  • Wastewater treatment in the aeration tanks with extended aeration
  • Secondary settling tank with hydraulic ability to collect scum
  • Tertiary treatment of wastewater in bioreactors using attached microflora
  • Nitrification and denitrification processes in wastewater plants

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Biological Wastewater Treatment System

Biological wastewater treatment is the foundation of sewage treatment in the  treatment facilities complex.

Biological wastewater treatment is carried out through destruction of organic matter by microorganisms under aerobic and anaerobic conditions.

Aerobic conditions offer the most efficient biological treatment.

The advantage of aerobic treatment is high speed and  low concentrations of additives. However, the disadvantages are high energy consumption and a large amount of sewage sludge.

Aerobic microorganisms are very diverse; most of them are bacteria that oxidize various organic substances independently.

Wastewater treatment through aerobic bacteria can occur under natural conditions in bio-ponds, in filtration fields. However, their widespread use is hampered by the seasonality, small capacity, and inefficient use of space. For this reason, in most treatment facilities including those designed and supplied by Bioxica, aeration tanks are used where efficiency of biological treatment is several times higher than under natural conditions.

To maintain the desired concentration of active sludge in the aeration reservoir, recycled active sludge from secondary settling tanks is used.

The attached microflora in bioreactors carry out the reactions.

Nitrification and denitrification are also used in biological wastewater treatment. These anaerobic processes remove nitrogen compounds and biologically oxidizable organic compounds.

Traditional biological treatment facilities such as aeration tanks and bioreactors can be used for nitrification and denitrification processes.

Wastewater treatment through anaerobic bacteria is useful if sewage is highly polluted by organic substances. Such treatment occurs in anaerobic reactors representing a container with minimum equipment. The activity of anaerobic microorganisms associated with release into the air of methane, which requires special vents. Typically, anaerobic treatment is combined with tertiary aerobic treatment methods.

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The most important components in sewage treatment plants of relatively small capacity (up to 7 000-10 000 m3 / day or 2 to 2.5 mg/d) are aeration tanks

In treatment plants Bioks, by Bioxica, extended aeration tanks are used. Depending on the design the aeration tanks can reach a depth of 8 meters (26 ft) or more.

Inside the aeration tank, the colonies of aerobic microorganisms (activated sludge) live on sludge flakes and process organic wastewater.

To promote the growth of microorganisms in the aeration tank, pressurized air is pumped to the aeration tank through mechanical, pneumatic or air ejector means (in Bioks plants the air is usually fed pneumatically).

As a result, the efficiency of biological treatment in the aeration tank is several times higher than under natural conditions.

In an extended aeration tank, the oxidation process occurs, as well as mineralization of hard dispersed substances.

It is important to maintain a constant concentration of activated sludge in the aeration tanks. To maintain its optimal concentration, part of the excess sludge is pumped in from the secondary clarifier.

With increasing doses of activated sludge in the aeration zone, there is a proportional growth of oxidizing power of aeration. However, concentration cannot exceed the point at which pollute drains on the final stage of purification.

To increase the efficiency of biological sewage treatment in the aeration tanks in Bioks plants, a plastic boot is used, which forms a biofilm.

Aeration tank in the residential wastewater treatment plant  Bioks "Tehnobridge-M, LLC "

In order to remove medium and fine impurities, settling tanks are used. They are grouped into constant and instantaneous action. Depending on the direction of wastewater flow, there are horizontal, vertical and radial settling tanks. Vertical settling tanks are used in wastewater treatment plants with a relatively small spillway capacity.

The settling tanks are divided into primary and secondary.

Primary settling tanks are located at the head of the treatment plant, and provide the primary wastewater treatment.

Secondary settling tanks are used for sludge separation and clarification of sewage after biological and physico-chemical treatment.

In Bioxica’s Bioks systems, vertical secondary settling tanks of continuous action are usually used. Wastewater flows here from the aeration tanks by gravity through a tray.

The lower part of the settling tank is pyramid or cone shaped so the settling sludge slides into the sludge collector.

Secondary settling tanks are an integral part of the production chain in the biological wastewater treatment. Part of the accumulated activated sludge is recycled to the aeration tank to create the required concentration of microorganisms. The remainder of the sludge is discharged into the sludge pit through the airlifts.

The design of the secondary settling tanks of treatment facilities Bioks by Bioxica provide automatic collection of floating scum into a special tray using a stream of water created by airlifts.

Secondary settling tank at the existing wastewater treatment plant Bioks by Bioxica

Secondary settling tank at the existing wastewater treatment plant Bioks by Bioxica

Tertiary treatment of wastewater in bioreactors using attached microflora

For tertiary treatment of wastewater, bioreactors (biofilters) with various types of loading are used. Biofilters with attached microflora are well proven. They are characterized by simplicity, convenience, and quick cleaning time.

In traditional biofilters, crushed stone, gravel, and expanded clay are used as filter material. However, plastic load has an operational and structural advantage over other materials. Plastic load can be hard (rings, trimming pipes, etc.), hard-block (flat and corrugated sheets), as well as soft (plastic films).

These materials are different in mechanical strength and durability. Plastic has a developed surface sorption and relatively low friction coefficient with the surface of the biofilm load holding unit volume compared with other types of material.

Plastic loading eliminates silting of biofilters and significantly increases the amount of inlet air, thereby increasing oxidative capacity of the filter.

However, biofilters have a number of disadvantages. For example, the uneven flow of wastewater from small objects affects the operation negatively. Drying out or uneven temperature also hurt microflora balance. In order to avoid these effects during minimum inflow, treated wastewater is recycled, which leads to additional energy use.

Bioreactor by Bioxica

The bioreactor supplied by Bioxica is a tank loaded with a plastic boot and gravel grit 2-10 mm.

Gravel grit is used to filter the remaining suspended solids after rough mechanical and biological treatment.

In the biological treatment process, nitrification and denitrification occur. They are the breakdown of  nitrogen compounds and biological oxidation of organic compounds.

Under favorable conditions (presence of oxygen, the temperature is above 4° C, etc.) in the first stage, nitrifying bacteria oxidizes the ammonium nitrogen salts. In the result, salt of nitrous acid, or nitrite, is generated first; in the second stage, during further oxidation, salts of nitric acid, or nitrates are formed.

The process of biological denitrification takes place under anaerobic conditions in the presence of organic substrate. Denitrifiers use oxygen-related nitrates and nitrites for the oxidation of organic matter, restoring them to molecular nitrogen, which is subsequently released into the atmosphere in the form of gas.

For the processes of nitrification and denitrification, aeration tanks and biofilters can be used.