The Karlovy Vary Sewage Treatment Plant is the biggest one in the area operated by the company Vodárny a kanalizace Karlovy Vary (Vodakva). It was built in years 1961 – 1967 and has been extended and modernized several times since then. The plant uses mechanical/ biological treatment process containing nitrogen and phosphor removal. The treatment capacity was designed for 80 000 PE. Nowadays, besides the city of Karlovy Vary, ten other surrounding municipalities are connected to this plant - it means about 60 000 inhabitants and a number of other wastewater producers (industrial or business premises). The average daily flow of this plant is about 24 000 m3 of wastewater which is treated and discharged into the Ohře River. The whole treatment process today is fully automated and computer controlled.

Treatment process

The plant´s technology is divided into three parts – mechanical stage, biological stage and sludge treatment. During the mechanical stage, the wastewater is pretreated and mechanically treated in order to remove all objects, sand or gravel, heavy solids and floating materials. During the biological stage, the dissolved biological matter is removed, including nitrogen compounds and phosphorus. The sludge accumulated in the wastewater treatment process is treated within the third part – sludge treatment.

Mechanical part of the process

The raw wastewater enters the plant by gravity via the main sewer and passes first through an overflow chamber which protects the plant from flooding. In case of heavy rain, the excessive water is collected in the rainwater storage tank located next to the sedimentation tanks and treated after the stabilization of the water flow. However in case that the water inflow is too intense and might damage the plant facilities, a part of the excessive water is discharged directly into the river. The overflow is equipped with mechanical screens to remove all large objects from the discharged water.

Pretreatment

The wastewater is then directed to the pretreatment unit which consists of two mechanical fine screens and a sand trap. The screens remove the impurities from the wastewater and the screenings are then washed and dewatered in the screening wash press. The sand trap is used to separate sand and gravel. The waste from pretreatment process is deposited at a landfill site.

Primary sedimentation

The wastewater continues on to the pumping chambers and then it is pumped into two large rectangular primary sedimentation tanks. The heavy solids (mostly of organic origin) settle here by gravity, the floating impurities are skimmed off and together they create the primary sludge which is driven towards the digesters.  The water is then directed to the second stage of the treatment – biological treatment process.

Biological part of the process

The biological treatment is the most important stage of the whole process. The undissolved matter is removed after the mechanical stage. Nevertheless, the wastewater still contains some dissolved and suspended matter, generated from household sewage or agricultural and industrial waste liquid. This matter is removed during the biological treatment process that uses water-borne micro-organisms (so-called activated sludge) in a managed habitat. During the treatment process this activated sludge goes through a cycle, when it is partly taken from the end of the process and directed back to the beginning. The biological stage consists of two activation tanks divided to several parts and six secondary sedimentation tanks.

Activation

Before the entrance to the first tank, the water is mixed with the return sludge from the end of the treatment process and also with a part of the activated mixture from the second tank in order to increase the efficiency of the treatment.  The first tank is used to remove nitrogen pollution in absence of oxygen. To obtain a homogeneous mixture of treated water and activated sludge the tank is mixed by mixers.

In the second tank the carbon pollution is primarily removed. The tank is aerated by blowers to inject oxygen used by microorganisms and to maintain the mixture composition.  Although the microorganisms partly consume the phosphates contained in water, it is not enough to remove all phosphorus from the water. Therefore, a coagulant is added to the water before entering the sedimentation tanks to generate precipitation and remove phosphorus from the water.

Secondary sedimentation

From the second activation tank, the water is directed into the system of six secondary sedimentation tanks, where the water is separated from the activated sludge. The mixture of bacteria settles down, light particles go up to the surface. The floating particles are skimmed off, the sludge from the bottom is removed and pumped back into the beginning of the biological treatment process. However, to reach the optimal rate of microorganisms, a part of this sludge is separated (so-called surplus sludge) and directed to the sludge treatment.
From the secondary sedimentations tanks, the treated water flows to an open channel which is directed into the center of the river Ohře.

Sludge treatment process

The sludge is the residue from the wastewater treatment process. The basic goals of the sludge treatment process are to reduce its volume and to stabilize the organic materials present in the sludge before its final disposal.

Thickening and digestion

The surplus sludge is first partly thickened by using a flotation thickener or centrifuge. Together with the primary sludge (from primary sedimentation), it is then directed into two digesters, where the sludge is heated in the absence of oxygen. During the digestion the organic solids are decomposed into stable substance and the biogas is produced which is used to generate heat and electricity in the cogeneration unit.

Dewatering and drying

The digested sludge is transported to the homogenization tank, where it is mixed to obtain a homogeneous mixture. Sludge imported from other small STP is also added to this tank. From the homogenization tank the sludge is pumped to three dewatering centrifuges, prior to the inlet a flocculant is added to separate more easily the water from the sludge. The sludge is dewatered to 23 or 27% of the dry matter.

The dewatered sludge is stored in the sludge silo and then it is pumped into the low-temperature belt dryer, where the sludge is dried by hot air convection. The final dried product (pellets) with a dry content between about 90% is then stored and transported for final utilisation.

Diagram of the sewage treatment process of STP Drahovice

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