GROUNDWATER TECHNICAL FACETS

clear water groundwater , , pre-filter , , post-filter , , reservoir , , water tower , , consumers

Figure 30 - Treatment of groundwater with double aeration/filtration

Example

As an example of the change in water quality of

deep anaerobic groundwater, the St. Jansklooster

pumping station is described. The treatment at this

pumping station consists of aeration, dry filtration,

aeration and submerged filtration. The values of

the different parameters in Table 4 are the average

values over a year.

As a result of the aeration phases, the amount

of carbon dioxide will decrease and the pH will

increase. Furthermore, the amount of oxygen will

Table 4 - QualitydataoftherawandtreatedwateratSt. Jansklooster pumping station (Overijssel)

Parameter

Temperature

pH

EGV

SI

Turbidity

Na-

K-

Ca'"

MgJ*

ci-HC03-

SO,*

NO,

°2 CH, CO,

Fe2-

Win2"

N H /

DOC

E-Coli

Bentazon

Chloroform

Bromate

Unit

°C

-mS/m

-FTU

mg/l

mg/l

mg/l

mg/l

mg/l

mg/l

mg/l

mg/l

mg/l

mg/l

mg/l

mg/l

mg/l

mg/l

mg/l

n/100 ml

MS"

M9"

M9/I

Raw water

10.5

6.9

51

-0.4

23

3

82

5.2

41

267

18

0.07

0

2

63

8.8

0.3

2.2

7

0

---

Clear water

10.5

7.6

48

0.2

<0.1

21

3

77

6.3

41

241

21

1.6

10.7

<0.05

11

0.04

<0.01

<0.01

6

0

---

increase. The concentration of Fe2 ' , Mn2 ' and NH4*

will decrease because of chemical and biological

transformations; the amount of nitrate, on the other

hand, will increase. The concentration of nitrate

increases less than the theoretical calculation.

4.5 Aeration and gas transfer

Aeration/gas transfer is the process by which water

is brought in close contact with air to change the

content of the dissolved gases in water. For the

treatment of groundwater, this means increasing

the oxygen content and decreasing the content

of carbon dioxide, methane, hydrogen sulf ide

and volatile organic compounds. The exchange

of gases that occurs in this process always takes

place simultaneously; aeration/absorption (gas

to water) and gas transfer/desorption (gas from

water). The close contact between air and water

which is necessary for aeration can be obtained

with var ious systems: by dropping the water

through the air in fine droplets (spraying), by divid­

ing the water into thin layers (tower aerators, cas­

cades), or by blowing small bubbles of air through

the water (deep well aerators, plate aerators, com­

pressor aerators). Technically, these systems can

be realized in a great number of ways.

The choice of a certain system is, to a great extent,

determined by the gases that have to be removed.

CH 4 and H2S have to be removed maximally,

because the remaining content will affect the post-

filters. The removal of C 0 2 has to be controlled

because its level influences the pH and with that the

SI. Table 5 gives a global indication of the effects

of the various aeration systems.

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TECHNICAL FACETS GROUNDWATER

Table 5 - Choice for a specific aeration system

Favorable effect

Input of 0 2

Low removal of CO?

Moderate removal of CO.,

High removal of CO?

High removal of CH4

High removal of H2S

Removal of mtcropollutants

Potential system

All systems

Compressor aeration, deep well aeration, cascades

Spraying

Tower aeration

High cascades, plate aeration, tower aeration

All systems, except compres­sor aeration

Tower aeration

Kinetics Gases are, to some extent, soluble in water. The concentration of a gas in the water phase is in equilibrium linear to the concentration of the gas in the air phase (equilibrium in conformance with Henry's Law). This equilibrium concentration is also called the "saturation concentration." Saturation means "at a fixed concentration of the gas in the air phase." This situation is obtained with a con­tinuous replacement of the air phase. For oxygen in water that is in equilibrium with air, a saturation concentration of 12 mg/l can be calculated (10°C, 21% oxygen, 1 bar).

The equilibrium isn't achieved at once with inten­sive contact between water and air. Rather, the gas exchange takes place at a rate which is linear to the driving force. This driving force is the differ­ence between the actual concentration and the equilibrium concentration:

= k2-(c,-c,) dc,

dt

in which: c, = concentration of a gas in water

at time t cs = equilibrium concentration of that

gas in water k, = gas transfer coefficient

(g/m3)

(g/m3)

(s-<)

0 400 800 1,200

time (sec)

saturation concentration concentration

Figure 31 - Change in the oxygen content at aeration in a particular system

For a certain aeration/gas transfer system, the gas transfer coefficient can be assumed to be constant.

Integration of this formula gives:

( C s ~ C l ) _ e - k , •.

( c . " c o)

in which: c0 = concentration of a gas in water

at time t = 0 (g/m3)

The profile of the oxygen concentration in an aera­tion device is shown in Figure 31. The oxygen content initially rises quickly and then increases less because of the reduced driving force.

Efficiency The efficiency of aeration can be defined as the achieved decrease in the driving force divided by the possible decrease in the driving force:

K = ( C

S - C ^ ) - ( C s - C o l = 1-e~

The gas transfer coefficient depends on the magni­tude of the contact surface between water and air (greater surface area, higher k2), and on the rate at which this surface is replaced (greater replace­ment, higher k ).

in which: K = efficiency of gas transfer (-) cm = concentration of the gas before

aeration (g/m3) cM= concentration of the gas after

aeration (g/m3)

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GROUNDWATER TECHNICAL FACETS

When aerating sequentially the efficiency will be equal per step, because the k2t-value of the same device is equal. This occurs in aeration with cas­cades, where the water makes the same falling motion at each step.

The efficiency of the aeration can be increased by increasing the k2t-value of the device (longer residence time, faster renewal of contact surface, increase of the turbulence in the water phase). The driving force can also be increased, by increasing the saturation concentration. For aera­tion this can be done by applying a higher operat­ing pressure (saturation tank, deep well aeration) or by applying pure oxygen instead of air. Gas transfer can also be executed with a lower pressure (vacuum gas transfer).

An aeration device has different k2t-values for dif­ferent gases.

Mass balance The amount of air needed for the addition of oxy­gen can be easily calculated. The volume of 1 mol air is 22.4 liters at 1 bar, so 1 liter of air with 21 % oxygen is (0.21-32 / 22.4 =) 0.3 g 02. To get 10 mg 0 2 in 1 liter of water, theoretically, only (10 / 0.3 / 1,000=) 0.033 I air is needed.

The transfer of a gas can be hindered by exhaus­tion or the accumulation of gas in the air phase. Ventilating the air can limit this.

According to the mass balance, the gas mass that is removed from the water is equal to the gas mass that is transported (carried away) in the air. Therefore, the air-water (volume) ratio (per time unit) is of importance:

RQ = ^ -

in which: RQ = air-water ratio (-) Qg = air (gas) flow (m3/s) Qw = water flow (m3/s)

For an increase in the oxygen content, an RQ of 0.5 is sufficient. This is (0.5 / 0.033 =) 15 times more than the amount stated above. To prevent

a limitation in the removal of CO.,, H,S or CH, 2 ' 2 4

a minimum RQ of 5 is necessary. When a high level of removal is desired, a higher RQ-value is obviously needed. This is, for example, the case when removing toxic, volatile compounds, like chloroform and trihalomethanes, which can occur in groundwater because of ground pollution.

In the case of bubble aeration the RQ-value is typi­cally smaller than 1. In the case of tower aerators, and plate aerators RQ-values of 10 and higher are applied. The differences in RQ-values cause, to a great extent, the differences in efficiency of these systems for specific gases.

Spraying By spraying the water against a solid body or against another spray of water, the water is dis­tributed over a large surface. Over the course of time, a great number of sprayers and spray methods have been developed. For the treatment of groundwater, the systems can be divided into:

- upwards or downwards spraying - spraying in a separate room or above a sand

filter.

With this, four combinations can be made, which are all being used (Figures 32 and 33).

When an intensive aeration/gas transfer is neces­sary, double spraying is employed, for example with a spraying room directly above a sand filter. The spraying floor functions as a reservoir for the

Figure 32 - Downward spraying with plate nozzles (Dres­den nozzles) over a sand filter at Schiermon-nikoog pumping station (Friesland)

274

TECHNICAL FACETS GROUNDWATER

Figure 33 - Upward spraying with Amsterdam nozzles in a separate room

first aeration and also as a dividing system for the

second aeration.

Most sprayers have a capacity of 1 - 3 m3/h per

sprayer, if 2 - 4 sprayers are placed per m2. With

this the surface load of spraying ( 2 - 1 2 m/h) is

approximately equal to the filtration rate of the post-

filters. Sprayers have a very limited flow range. In

the case of low flows, there isn't a good distribution

of the water and a poor aeration results. Therefore,

to deal with large fluctuations in capacity one can

close off a number of sprayers. The energy used

for spraying is limited (0.5 - 2 mwc to provide pres­

sure drop in sprayers and 1 - 2 m as fall height, so

1 . 5 - 4 m in total).

Spraying is efficient for the addition of oxygen and

the removal of CH 4 (for both 80 - 90%), but less

efficient for the removal of CO., (40 - 50%).

The addition of oxygen results, almost directly,

in the oxidation of dissolved iron. Therefore, on

the walls and pipes of the spraying systems, iron

deposits can be found. This pollution can be lim­

ited by using downward spraying directly above

the filter. Pollution will always occur in the dividing

system and in the sprayers, which will affect a good

distribution during spraying. Spray systems will

have to be cleaned a few times per year. An open

division floor is attractive because of the acces­

sibility of the parts that have to be cleaned.

In the case of dry filtration, spraying is always used

to distribute the water evenly over the filter bed.

The spraying room has to be ventilated with a flow

that results from the desired RQ-value for the aera­

tion/gas transfer. This air is filtered in advance to

Figure 34 - Cascade aeration with distribution from many jets at Heel pumping station (Limburg)

avoid contamination of the microbiologically reli­

able groundwater by contaminants in the atmos­

phere (aerosols, etc.).

Cascade or waterfall aeration

In cascade or waterfall aeration the water is divided

into a thin layer by letting it fall over a sharp edge.

In past years many different types of cascades

have been developed. In the most efficient types of

construction, the water layer is divided into various

spouts that fall into a water trough. During this, air

bubbles are forced into the water and, because of

the turbulence, these separate into many small air

bubbles (Figures 34 and 35).

The water trough has a minimum depth of 6 6 % of

the fall height to obtain sufficient contact time. The

rate of the water depends on the fall height:

Figure 35 - Principle of the formation of air bubbles in cascade aeration

275

GROUNDWATER TECHNICAL FACETS

' = V2-g-h

in which: v = fall rate h = fall height g = gravity acceleration

(m/s) (m)

(9.81 m/s2)

The efficiency of a cascade depends little on the hydraulic load (up to 200 m3/h per m). Therefore, cascades are a very robust way of aerating, inde­pendent of fluctuation in the production capacity and barely sensitive to deposits of iron on the cascade.

With a height between 0.5 and 1.0, the fall rate is between 3 and 4.5 m/s. With a spout thickness of 1 cm and a fall rate of 3 m/s, the capacity of a cascade is (0.01 • 3 • 3,600 =) 108 m3/h per m overflow length. The efficiency of the addition of 0 2 and the removal of CH,, is more or less linear to the fall height (approximately 50 - 60% per m fall height). Unfortunately, C02 is very poorly removed in a cascade (approximately 10 - 20% per step, regardless of the height).

In practice a maximum fall height of 1 meter per step is chosen. The efficiency barely increases above this height. A higher efficiency can be obtained by using more steps. For a very thorough removal of CH4 cascades with 4 - 5 steps and a total fall height of 4 - 5 m are used.

Bubble aeration Bubble aeration for groundwater treatment occurs in three types of facilities: - compressor aeration - deep well aeration - plate aeration

Bubble aeration has the advantage that, it can be easily incorporated into the treatment scheme. Namely, in the case of bubble aeration, the neces­sary energy isn't extracted from the water phase (decrease of potential energy), but given to the air phase. This makes the addition of energy and, therefore, the gas transfer much more flexible than the other aeration systems. Besides, compressor or deep well aerators are used when the removal of CO, is undesirable.

Many small steps are used if the largest possible CO, removal is desired.

Compressor aeration In the case of compressor aeration, air is directly injected into a water pipe through a compressor.

Calculation for cascade aeration

In the first step of a cascade aerator the oxygen content increases from 0 to 4 mg/l. Calculate the oxy­

gen content after the subsequent steps.

With a saturation concentration cs of 12 mg/l, the oxygen efficiency of a single step of a cascade can be calculated as follows

( 4 - 0 ) / ( 1 2 - 0 ) = 0.33

Thus, per step k21 = - ln(1 - 0.33) = 0.40 With a residence time of 5 seconds per step is k2 = 0.40 / 5 = 0.08 s"

After the second step the oxygen content is (4 + 0.33 • (12 -4 ) = 4 + 2.6 =) 6.6 mg/l After the third step the oxygen content is (6.6 + 0.33 • (12 - 66) = 6.6 + 1.8 =) 8.4 mg/l After the fourth step the oxygen content is (8.4 + 0.33 • (12 - 8.4) = 8.4 + 1.2 =) 9.6 mg/l

The efficiency per step remains at 33%, but the absolute increase in the oxygen content decreases more and more (from 4 mg/l per step to 1.2 mg/l per step).

276

TECHNICAL FACETS GROUNDWATER

Figure 36 - Venturi aeration, a version of compressor aeration

By a narrowing of the pipe, a higher turbulence is

obtained in the water, leading to small air bubbles

and good mixing.

For a fast gas transfer it is preferable to inject air

at a place where the water has a high pressure,

but, when looking at the energy consumption of

the gas phase, this is undesirable.

With a pressure of 3 mwc in the water pipe and an

RQ of 0 . 1 , the energy consumption of the compres­

sor aeration is (0.1 -3 =) 0.3 mwc. For the mixing,

another 0.5 mwc is needed.

With this system, oxygen can be added, but gas

removal almost never occurs.

Compressor aeration is mostly used in combina­

tion with pressure filtration.

Venturi aeration is a variant of compressor aeration

and was often applied in the past (Figure 36). The

air is entrained by a vacuum in the venturi.

Of course this way of aerating is very sensitive

to variations in flow, because the vacuum varies

with the square root of the water rate in the throat

of the venturi.

Deep well aeration

For deep well aeration, air is brought into a vertical

tube, where the mixture flows down to a depth of

5 to 20 m (Figure 37). Here, the water flows out of

the tube into a larger shaft. Because of the high

hydrostatic pressure, a very high transfer of oxygen

is achieved, within an exceptionally small space.

Furthermore, the energy consumption is very low.

The air is injected almost under atmospheric pres­

sure, which consumes little energy. The energy

consumption of deep well aeration is mainly the

hydraulic resistance of the vertical tube. This is not

completely negligible because a rate of 2 - 3 m/s

A supply raw water B air supply C outlet for aerated water

Figure 37 - Deep well aerator

is necessary to carry along the large air bubbles.

Furthermore, the water in the vertical tube has a

smaller specific gravity due to the air bubbles and

therefore, also a pressure difference will have to

be realized to obtain a net gradient.

Oxygen can be added to this system, but gas

removal rarely occurs. From a deep wel l over-

saturation of gases can occur and, in the post-sand

filters, gas transfer can occur. This may result in

faster clogging of the filters.

Plate aeration

With plate aeration, a large amount of air (RQ

of 30 - 60) is blown through a thin layer of water

(ca. 25 - 30 cm). The air is injected through many

small holes (ca. 1 mm) in the bottom (Figures 38

and 39). The large amount of air causes strong

turbulence and a good gas transfer, despite the

short residence time ( 1 0 - 2 0 s).

2 7 7

GROUNDWATER TECHNICAL FACETS

water

Figure 38 - Principle of plate aeration

The energy consumption in the water phase is very low (slow flow in an open trough), but in the air phase the energy consumption is very high. With an RQ of 50 and a water height of 0.30 m, the energy consumption is (500.30 =) 15 mwc.

With this system a very good methane removal is possible (90 - 95%) and considerable C02 removal (60 - 70%). This system is definitely used if extra methane removal is necessary afterwards, and if there is no hydraulic gradient available. The system is very sensitive to fouling, meaning that the small holes can clog due to iron and calcium deposits. Therefore, cleaning of the bottom plates is necessary several times a year. To limit the amount of outside air that has to be filtered, recir­culation of used air is warranted.

Tower aeration In the case of tower aeration, water is distributed over a column with packing, through which air is blown (Figures 40 and 41).

Tower aeration is easily configured for the removal of gases by: - choice of bed height (1 - 5 m) - choice of packing material (course, fine, open,

filled)

^J

Figure 39 - Plate aeration at Oldeholdpade pumping station (Friesland)

A influent B packing C air supply D effluent E air outlet

Figure 40 - Principle of tower aeration

- choice of air direction (co-current, counter-cur­rent)

- choice of RQ(1 -100)

Due to such design choices, a very good removal of gases is possible. Tower aeration is not very sensitive to variations in hydraulic load (50 -150 m/h). It is, however, sensitive to fouling, which makes regular replacement or cleaning of the packing material necessary if the water contains iron. In some cases this cleaning is performed by backwashing the aeration tower.

Tower aeration is also sometimes realized in a pressure vessel. With a vacuum, gases can be removed without gases being added to the water (vacuum gas transfer).

With high pressure, over-saturation can occur. This is used in saturation for flotation.

4.6 Filtration Filtration of groundwater is a process similar to the filtration of surface water. Both consist of a filter (sand) bed of 1 - 2 m through which water flows. The filter is backwashed (almost) daily, cleaned

278