Brno – Modřice WWTP

The company is in association with

Life2Water

Waste Water Treatment Plant Brno - Modřice

Intruduction

The wastewater treatment plant in Modřice treats wastewater conveyed by a system of sanitary sewers from the city of Brno and increasingly by a system of pumping stations from the wide surroundings of  Brno. At present, the towns of Kuřim and Modřice, and the municipalities of Želešice, Česká u Brna, Šlapanice, Šlapanice-Bedřichovice, Ostopovice, Moravské Knínice, Lipůvku, Podolí, Ponětovice a Rozdrojovice (see the sewerage layout) are connected to the WWTP besides Brno.
The original Modřice was put into operation in 1961 as a classic two-stage plant with anaerobic sludge stabilisation. With the city development accompanied by growing hydraulic and mass overloading  it was necessary to gradually extend the whole plant in the 1980’s.
The overall reconstruction and extension of the WWTP was prepared from the year 1992 and the need of its implementation was becoming more and more urgent. In order to do so the company along with the City of Brno made every effort to raise funds necessary to reconstruct the existing parts of the plant and to extend the plant. This intention was executed at the end of 1999 when a Loan Agreement was signed with the European Bank for Reconstruction and Development. This was a pre-condition for the commencement of the project. The objective of the Modřice project was to meet the treated wastewater effluent limits set by Czech and European standards and regulations, and to ensure sufficient capacity of the facility to accommodate the growing demand of the city of Brno and the surrounding agglomerations for connecting to the Brno sewerage system conveying wastewater to the  Modřice WWTP.
Based on the results of competitive bidding, the project of reconstruction and extension of the wastewater treatment plant for the city of Brno in Modřice  was awarded to a consortium consisting of a French leader –Degrémont – and a joint venture of construction companies  IMOS, a.s. and ŽS. Another member of the consortium was the designing company  AQUATIS, a.s. At the end of May 2001, the construction work was initiated and it was completed at the end of the year 2003, when the testing operation permit was applied for. The testing operation commenced on 1st January 2004. Completion of the testing operation with satisfactory results was confirmed by the certificate of completion followed by take-over of the plant by the employer an the completed plant is now in permanent operation. 


Permissible volume of discharged wastewater:

Qmax. = 4 222 l/s                                               Q bil. = 61 520 m3/year

Permitted values of residual pollution at the WWTP effluent:

 

„ efficiency“

„balance“

COD

85%

3322 t.rok-1

BOD

95%

615 t.rok-1

Ntot.

75%

615 t.rok-1

Ptot..

85%

3322 t.rok-1

 

 

 

 

 

„avg“

  „max“

„balance“

  NL

20 mg.l-1

40 mg.l-1

923 t.rok-1

 

 General view

 

Raw wastewater is conveyed to the wastewater treatment plant via an inlet structure used as a flow split. During rain events, the influent to the WWTP is limited to Qmax. = 4.222 m3/s. If the flow rate is higher, the rainwater is first collected in a stormwater tank with a capacity of 10,500 m3 and hydraulic clearing. After the rain event, the water collected in the stormwater tank is pumped back to the WWTP. Water coming to the WWTP is pre-treated and freed from coarse gravel in a gravel trap and then it passes mechanically scraped screens with slot width of 6 mm. Screenings from the screens are pressed and then flushed with water. Water flows from the screenings room by gravity to an aerated sand trap equipped with grease separation.  The sand is further processed in a sand classifier and washer and then stored in a skip.  Water coming from the gravel trap and the sand trap is conveyed by a channel to a screw pumping station  from where it is pumped to a flow split dividing the flow into six primary clarifiers. During dry weather flows, the process includes a maximum of four primary clarifiers, the remaining two are used during rain. The primary clarifiers ensure mechanical removal of settleable particles.  The existing tanks and flows plots of these tanks have been refurbished (see the picture – mechanical stage).
Following the mechanical treatment, the wastewater is conveyed by a pipe to an intermediate pumping station from where it is pumped by four pumps into activation. The activation is divided into two lines, each with two separate lanes that can be operated separately or together. Water is first conveyed to the anaerobic tank (phosphorus removal), then to a circulatory anoxic serving for pre-denitrification. The last stage of activation is an anoxic part with fine-bubble aeration divided into aerated and unaerated zones. Air is supplied from the reconstructed blower house by four blowers. Return sludge from the secondary clarifiers is freed from nitrates by denitrification in the pre-anoxic zone located in the first part of the activation in order to achieve efficient phosphorus removal. Phosphorus removal ensured primarily by the biological process; however, it is also possible to dose ferric sulphate in order to achieve the required results.
The mixed liquor from the activation tanks is fed into six secondary clarifiers where activated sludge is settled and separated. Settled sludge is conveyed via a return sludge pumping station to the pre-anoxic zone of the activation. Activated excess sludge taken from the activation is processed in the sludge management system (see the picture – biological stage).
The sludge line consists of a primary sludge thickener, DAF flotation thickener for the  biological sludge, mechanical GDD screens, homogenisation tank, digesters, digested sludge storage tanks, sludge dewatering plant and sludge drier. Primary sludge from the primary clarifiers is thickened using a classic gravity thickening tank of a circular type and extracted into a sludge mixing tank. Clarified water returns to the primary clarifiers flow split. Excess biological sludge conveyed from the  biological stage is thickened in the DAF flotation unit. 3 units of mechanical GDD screens are used as a back-up for primary and secondary sludge thickening. These two types of sludge are mixed in a homogenising mixing tank equipped with mixing. Mixed raw sludge is then pumped to the digesters.  Fibres are removed from the primary sludge line by pre-filtration. The sludge in the four existing digesters is intensively mixed  and kept at a constant temperature of 35°C in order to ensure growth of mesophilic  bacteria. The sludge retention time in the digesters is approx.  22 days. The remaining two former digesters have been reconstructed to serve as stabilised sludge storage tanks with a useful storage capacity for more than four days. Digested sludge from the storage tanks (dry solids content of approx. 4%) is dewatered by two centrifuges type Guinard. One centrifuge is used for dewatering, the second is used as a back-up. Sludge with dry solids content of 24% is transported by a screw conveyer to the sludge drier. The paddle sludge drier type NARA is based on a system of indirect sludge heating. Heat in the sludge drier is transmitted by thermal oil  (180-210 oC) flowing inside the jacket, hollow shafts and paddles. Long sludge retention time  (over three hours) combined with the average sludge temperature  of  100 oC make it possible to provide pasteurisation and sanitary treatment of sludge (see the picture – sludge drier). Dried sludge with  90 – 92 % d.s. is conveyed from the drier by means of cooled conveyers into two skips located outside the sludge drier plant  from where the filled up skips re moved to a dried sludge storage room (see the picture – sludge management).
Biogas produced during sludge digesting is extracted from the digesters and collected in two diuble-membrane gas holders with a total volume of 3,000 m3 and then used for el. power and heat generation  in cogeneration units with a throughput of  2 x 500 kW.  The biogas is freed from hydrogen sulphide in a desulphurisation unit. Excess biogas is burnt is residual gas burners   (see the picture – gas management).