Deya Kavala (Contributed by Nama & Deyak)
The seaside city of Kavala is the capital of the Prefecture of Kavala and is situated in northern Greece some 165 km east of Thessaloniki, the capital of Macedonia. The responsibility for both water supply and sewage disposal rests with DEYA Kavala, a company wholly owned by the municipality, which was established in 1984. DEYAK has currently 111 personnel in two principal departments. The Technical Services Department has 86 people and the Administration/Accounts Department has 25 people. The population which is currently served by DEYAK is estimated at 75,000. The population is estimated to increase by 5000 during the summer due to tourism, thus giving a total of 80,000 people.
Historical Development
The water supply system for Kavala was originally developed early this century and the town was supplied for many years from a Roman aqueduct transferring water from springs 5.5 km north of the town at an elevation of 400 m A.S.L. The yield of the springs varied greatly, reducing to 600 mVday during the summer and due to the poor condition of the aqueduct, only half of the yield eventually reached the town.
By 1928 the aqueduct was repaired and the town was supplied from the following sources:
a. Springs 5.5 km north of the town (25-40 mVhr)
b. Two boreholes 1.5 km west of the town ( 6-8 m3/hr, which suffered from salinity intrusions during periods of overpumping)
c. One borehole 1.5 km east of the town ( 7-8 m3/hr).
There was no general distribution system in Kavala at that time, apart from a limited number of small diameter steel pipes carrying water from reservoirs to public fountains.
In 1928, four boreholes were sunk to a depth of around 50 m by the old Kavala to Thessaloniki national road, 7 km northwest of the town. Water was pumped from a level of 50 m A.S.L., via a 3 km 400 mm cast-iron main, to a reservoir near the town (at an elevation of 199 m A.S.L.) from which it was fed to various reservoirs in the town. Only two of the boreholes were used and the pumping rate was 235 nrVhr. In 1947, what is now known as the "old" pumping station was constructed and the oil engine pumps were replaced with electrically driven units. Only one borehole was used and according to the venturi meter the flow was 200 m3/hr.
During 1957-1958, the level in the boreholes fell dramatically and thus only intermittent pumping was possible. The problem was overcome by sinking two more boreholes nearby which were pumped alternatively giving a flow of 200 mVhr. From 1961 onwards wet periods replenished the underground aquifer but by then it was realised that the aquifer was not capable of yielding more than 250 to 300 m3/hr without the risk of lowering the level in the boreholes, especially during dry years. This initiated a search for new sources and a study undertaken in 1963 identified the Voirani springs 25 km from Kavala as a possible new source.
Current System - Overview
The Voirani springs are now the primary raw water source for Kavala with the older boreholes providing a secondary source. Raw water from the Voirani springs gravitates to the Main Pumping Station located near to the borehole field and adjacent to the Reserve Pumping Station. Water is treated (disinfection only) at the pumping stations and discharges either to a balancing reservoir some 3km distance at the head of a tunnel, leading through into the main catchment of the city itself, or to a header tank at Ag. Sillas. A number of service reservoirs are located around the perimeter of the city, interconnected by transmission pipework and mostly fed by gravity from either the balancing reservoir or Ag. Sillas header tank. A third main pumping station, high zone, draws from the gravity transmission main and delivers to high zone reservoir which supply the higher parts of the city.
Voirani Springs Hydrological Investigations
Several previous hydrogeological studies have concluded that rainfall recharge is insufficient to account for the flow of these springs, and that some other source, such as the river Nestos, located some 35 km to the north-east, must be contributing to the spring. In order to model the effects of climate change and other hydrological extremes and their influence on the water supply system of the town of Kavala, the source of the Voirani springflow has be ascertained.
A) Catchment area of the Voirani springs
The Voirani springs arise from a karst marble aquifer with a fairly unique and locally complex structure. Physical and geological factors suggest that the springs are at the lowest point in a sub-basin, and the south-westerly ground-water springs to the surface here because of a set of barriers: a fault, an anticlinal axis and a bed of impermeable gneiss.
Recharge of the marble aquifer is derived from rainfall on its outcrop ('direct recharge') as well as runoff from the adjacent gneisses ('indirect recharge'). Geological and topographic maps define the surface and ground-water catchments of the Voirani springs. The surface-water catchment is larger than the ground-water catchment. The additional area is mainly underlain by gneisses. Rainfall on these impermeable gneisses should produce runoff that subsequently recharges the marble aquifer along the lower reaches of the drainage network. Using estimated areas of the marble outcrop and the gneiss, and estimated recharge figures along with average annual rainfall figures, it appears that the Voirani spring flow can be accounted for by rainfall recharge alone on the aquifer.
B) Link with river Nestos
It has been suggested that 65% of the flow of Voirani springs is derived from the river Nestos. This estimate depends on the definition of the catchment area of the springs and the proportion of annual rainfall contributing to recharge.
The river Nestos enters the area from an extensive region of thick and impermeable gneisses to the north-east. However, only a minimal stretch of its reach which runs over the marble aquifer has an elevation greater than that of Voirani springs; any recharge further downstream would not contribute to the springs due to lack of an hydraulic gradient.
Comparing flow profiles of the river Nestos and the springs, we can begin to understand where the recharge originates. The Voirani springs do not exhibit a typical recession curve from the winter maximum. Instead, the flow is maintained at a high level until about June. This may be due to a combination of a large catchment area (also suggested by its large flow), inflow from more distant parts of the basin (which could include river recharge) or rainfall recharge during spring/early summer.
The main structural trend in the area would seem to favour a general ground-water flow from the river Nestos towards Voirani springs. The topography and the likely water-table elevations suggest that the river Nestos is gaining water from the adjacent marble aquifer, and that the water-table has a higher elevation in the area between Voirani springs and the river Nestos.
The gneiss ridge, to the north side of which the Voirani springs are located, extends to within a few kilometres of the Nestos. High level springs are common in the gneiss/schist outcrops but these are due to local, perched aquifers, and the gneisses/schists can be considered as impermeable. Thus, the gneiss ridge probably forms a barrier to ground-water flow between the Nestos and Ksiropotamos basins.
Not only do the local conditions suggest that a significant link between the river Nestos and Voirani springs is unlikely, but the flow of Voirani springs can be accounted for by recharge in the Ksiropotamos valley based on a preliminary estimate of the contributing area and annual recharge.
C) Maintenance of Supply
Currently, the town of Kavala has enough water for its needs. However, DEYA is concerned about the long-term stability of the Voirani springs. Any events, whether natural or man-made, which could affect the spring-flow, threaten to adversely affect the water supply system of Kavala.
The main issue of concern to DEYAK is the security of supply from the Voirani Springs due to the following reasons :
• conjunctive use of the springs ( DEYAK, irrigation, communities )
• reduction in the yield of the springs during drought periods
• the river Nestos
The Nestos river flows on the other side of the aquifer from which the springs stem and many studies have suggested the existence of a hydrogeological link between the river and the springs. Bulgarian and Greek authorities are planning significant hydropower schemes on the river and these could disrupt the level and pattern of flow in the river, which in turn could threaten the outflow from the springs. However, as stated above, a preliminary investigation undertaken indicated that a significant link between the River Nestos and the Voirani springs is unlikely and that the flow of the springs can be accounted for by rainfall recharge.
Water Resources Management Of The Voirani Springs
The flow at the Voirani springs has been monitored by the Ministry of Agriculture
The following users are supplied from the Voirani springs :
a) DEYAK
b) the Voirani irrigation network ( 67,100 hectares )
c) the water supply networks of the Associations of Doxato and Ag. Paraskevi
communities
communities
DEYAK has a legal agreement which states that the maximum quantity that can be utilised from the springs is 460 1/s. DEYAK currently abstracts less than that, but the exact quantity is not known as no flow measurement is undertaken at the springs. However, an estimate of this quantity can be made from the number of pumping hours at the main pumping station. Based on data from 1995, average monthly flows can be calculated.The quantity of water utilised by the two Associations is estimated at 80 1/s. Data on irrigation requirements are abstracted from a report compiled by the Ministry of Agriculture and they refer to 1990 data.
Problems Faced By Deyak
It is noted that the total yield of the Voirani springs and the boreholes would be adequate for the needs of existing users ( DEYAK, irrigation, communities ). However, the legal agreement between DEYAK and other users of the source, does not safeguard the continuity of supply during periods of drought, increased irrigation demand and in the event of another potential user emerging. It is also a feature of Greek environmental legislation that there is no supervisory body to licence and control abstractions from springs or groundwater and this sometimes results in conflict between irrigation and water supply needs.
Leakage levels are considered to be high and according to the annual quantities of produced and billed water, the percentage of unaccounted for water in 1995 was 59%.
Development Of The Water Resources Management Model
It is proposed that the model which will be developed, will include the following components :
Voirani springs, the demand zones of communities, DEYAK and irrigation network, the group of boreholes supplying the irrigation network and all pipes interconnecting them
Model Applications
The water resources management model will be used to : simulate the yield of the Voirani springs for scenarios of climatic change assess the effects of climatic change scenarios on current users of the Voirani springs ( DEYAK, irrigation network, communities, industries ) assess the effects to the water resources available to DEYAK from the springs in the case of increased irrigation demand during drought periods or as a result of expansion of the irrigation network assess the feasibility of utilising other sources in the following cases
a) reduction in the yield of Voirani springs due to climate changes
b) increased irrigation demand
investigate the feasibility of carrying leakage control in order to offset any reduction in the available water resources due to climate changes or increased irrigation demand and in order to decrease pumping costs
KAVALA SEWAGE TREATMENT PLANT
The sewage led to the plant is pumped into the plant by a pumping station located outside the plant site.
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, 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 aprox. 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:
1) to keep the bacteria alive
2) to decompose the organic matter
3) to allow the nitrification
Roughly, approx. 1/3 of daily oxygen consumption, constituting approx. 8,000kg O2, is used for each of the 3 purposes.
As the gentrification process allows decomposition approx. half of the organic matter by means of nitrate and not free oxygen, a saving of approx. 1,500kg O2/d is obtained.
Consequently, the rotors only have to provide approx. 6,500kg O2/d (at 80,00 PE) when denitrification takes place. In other words, it is cheaper to include denitrification than just to nitrify the sewage.
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