Kavala Sewage Treatment Plant
The sewage is introduced into the plant at a by-pass well. From the by-pass well the water gravitates to a mechanical screen with a hydraulic rake, where large objects are retained. The screenings retained in a container are taken to a refuse dump.
From the screen structure the water is taken to an aerated grit chamber designed with 2x3 pits where the grit settles. The settled grit is pumped to a grit separator adjacent to the grit chamber where it is drained. The dry grit retained in a container is taken to a refuse dump. The grease is separated into a grease chamber and taken to a grease well for removal by an exhauster.
From the aerated grit chamber the water gravitates to a flow measuring chamber equipped with a Parshall flume where the water flow is measured and recorded. From the flow measuring chamber the water runs to a by-pass well and continues via inlet distribution weirs to the oxidation ditches for biological treatment.
The oxidation ditches are two identical structures. The water is introduced into one of the ditches from where it continues to the other ditch before leaving the oxidation ditches via mechanically operated overflow weirs.
From the oxidation ditch, the biologically treated sewage is taken to clarifiers via a distribution chamber. There the content of activated sludge in the sewage is settled, and the treated water proceeds via an overflow at the periphery of the tank to the outlet chamber. The function of the outlet chamber and the outlet pipe is not included in this manual.
The actual treatment is performed in the oxidation ditches as an activated sludge process with extended aeration, including nitrogen removal (the so-called Bio-Denitro process).
The settled sludge is taken to a sludge pumping station by means of a sludge scraper in the clarifiers. The scraper brings the sludge into a centre cone from where it is led to the sludge pumping station. The sludge pumping station has two functions: one for return sludge pumping and one for excess sludge pumping.
The return sludge is pumped back to the by-pass well at the inlet end of the oxidation ditches to maintain a constant amount of sludge in the oxidation ditches.
The excess sludge is pumped to a concentration tank for concentration before it is pumped to a homogenizing tank. Air is blown into this tank to make the sludge homogeneous and thus improve its dewatering properties.
Finally, the sludge is pumped to belt filter presses where the sludge will be pressed between two filter belts and dried. Chemicals are added to the sludge before it enters the presses to improve the dewatering process.
The dried sludge is retained in containers and taken to a refuse dump.
The plant includes a washwater pumping station which pumps the treated wastewater from the clarifiers back into the plant for cleaning purposes, including cleaning of the belt filter presses.
Furthermore, the plant includes two scum wells to collect the floating matter from the surface of the clarifiers. The scum from these wells is removed with a mud exhauster.
Dimensioning Data
The sewage treatment plant is dimensioned for the following loads:| Stage I | Stage II (year 2020) |
|
| Population | 80,000 PE | 120,000 PE |
| BOD5 daily (kg) | 4,800 | 7,200 |
| Nitrogen daily (kg) | 800 | 1,200 |
| Suspended Solids kg | 5,600 | 8,400 |
| Wastewater flow: | ||
| Average per day (m3) | 12,000 | 24,000 |
| Max. per hour (m3 ) | 750 | 1,500 |
| Max. per hour incl. | ||
| storm water (m3 ) | 1,500 | 3,000 |
Generally speaking, most of the plant units are dimensioned for loads corresponding to Stage I, with provisions for easy future extension. Some units such as Screen Structure, Parshall Flume, Distribution Chambers, By-Pass Wells, Concentration Tank, Homogenizing Tank and interconnecting pipes are dimensioned for loads of Stage II (year 2020).
It is assumed that:
the water taken in does not contain matters which are harmful to the biological processes such as organic solvents and the like,
the pH-value of the influent is in the range between 6.5 - 9,
the alkalinity of the water is sufficient to maintain a pH-value above 7 to avoid the addition of lime,
the inflow is min. 40,000 PE for the sake of the denitrification process,
the sewage treatment plant is maintained according to the instructions.
Based on the above assumptions the plant is dimensioned to meet the following effluent quality demands:
| BOD5: | 25 mg/1 |
| Suspended solids: | 30 mg/1 |
| Total N (Inorganic): | 8 mg/1 |
| NH*-N: | 2 mg/1 |
| COD: | 100 mg/1 |
Pretreatment
The pretreatment section removes the substances that may cause aesthetic nuisances in the recipient. Furthermore, the mechanical pretreatment is important to protect the treatment plant.The screen and the grit chamber remove objects and particles which may cause clogging and wear and tear of pipes and pumps, deposits in tanks (especially oxidation ditches) and obnoxious smells. The grease chamber retains grease and oil which may have an inhibiting effect on the biological process, cause obnoxious smells, clogging, accumulation in sludge, foaming in oxidation ditches, danger of explosion, corrosion etc.
Biological Treatment
An activated sludge plant comprises an oxidation ditch, clarifier and return sludge pumping station.Activated Sludge
Activated sludge consists of a constant amount of microorganisms in the form of floes (aggregates) which decompose organic matter by means of oxygen (or nitrate) and produce excess sludge. The excess sludge is extracted from the plant at regular intervals.Bacteria
Out of the many types of bacteria living in air and water, there are 3 types present in the 3 processes. These bacteria use the pollutants of the sewage as "food" (organic matter and ammonia) and by means of oxygen or nitrate, harmless gases are formed which abound in the atmosphere already (carbone dioxide and nitrogen). Part of the pollutants are incorporated in the excess sludge and the small part remaining leaves the treatment plant with treated sewage.The biological processes depend on nature of the sewage, the amount of oxygen, mixing of food, oxygen and bacteria, acidity (ph), any inhibiting substances like chrome and copper and especially temperature. If the water temperature is 25o C, the processes takes place twice as quickly as at 15o C.
BOD – Biochemical Oxygen Demand
In a sample taken from the influent to the treatment plant, the organic decomposition will be started immediately by one bacteria type.The amount of oxygen consumed is expressed as BOD – the Biochemical Oxygen Demand. Every day, one person produces a certain amount of organic pollution corresponding to approx. 60g BOD – called one PE (population equivalent).
The nitrification does not start until a few days later (after approx. 5 days at 15o C) by means of another type of bacteria. To enable the nitrification process in a wastewater treatment plant, the bacteria contained in the sludge must have a certain minimum age – called sludge age.
Excess Sludge Amount
The excess sludge amount is determined by the amount of food removed from dewage by the bacteria. The amount of food (organic matter) is expressed as the amount of oxygen (kg BOD) required for the decomposition.Sludge Age and F:M – Food to Micro organism
To obtain sufficient purification of the sewage, a retention time of some hours is required in the oxidation ditch (in this case approx. 16 hours).If the sludge is to be stable, it must be retain in the activated sludge plant for some days before being extracted for dewatering (in this case a sludge age of approx 10 days). The expression for this is extended aeration.
Oxygen Consumption
The function of the rotors is to oxidate the water for 3 purposes:- to keep the bacteria alive
- to decompose the organic matter
- to allow the nitrification