BIOLOGICAL FILTRATION OF WELL WATER FOR POTABILISATION

Introduction

Intensive exploitation of the deeper strata, consequently upsetting the age-old equilibrium, the laws concerning drinking water quality and the refinement of water investigation techniques have led, in recent decades, to the identification of contaminating substances in the deep water, previously not considered as such or not present or, if present, not identified.

These are generally reducing substances that have formed in the substantially anaerobic environment of the deep water such as: ammonia, hydrogen sulphide and methane; if we add to these substances iron, manganese, carbonic acid and humic or humic-like substances, we have a complete picture of the majority of deep water available for inclusion in the drinking water network.
For some time now, it has been common practice to eliminate the iron, manganese, carbonic acid and humic substances using techniques that are consolidated but which are not always sufficient to reach the low limits imposed by the recent law.
Experimentation which started in the 60s, initially controlling well water treatment plants and, subsequently, in our laboratories and on a semi-industrial scale, on pilot plants specifically constructed for the purpose, has led to development of the so-called "biological filtration" technique.

Process description

The process consists substantially in exploiting the work of a mass of aerobic micro organisms fixed on a bed, crossed by the water to be treated and in the presence of oxygen. The community of micro organisms is selected to specialise in obtaining energy, necessary for its growth, from oxidisation of the various compounds, both organic and inorganic, present in the well water. In this way the ammonia is oxidised into nitrate, the hydrogen sulphide into sulphate, iron and manganese into insoluble hydroxide and dioxide respectively, while the methane is oxidised into carbon dioxide and water. The humic substances can serve both as "pabulum" for the micro organisms (in which case they are demolished) and as "temporary" support (in which case they are not modified).

The quartzite biological filter functions firstly as a physical filter: the water still rich in iron oxides and other insoluble salts will be fed from the top of the fluidised bed where it will begin to deposit part of these solids: as the filtration proceeds through the lower layers, the iron oxides deposited on the quartzite granules will act as aggregation sites for "capturing" further oxides and salts, thus permitting total removal of the iron (in the form of insoluble oxides) right from the first few metres of the filtering layer.

Biological oxidisation takes place in the subsequent layers of the quartzite filter. The well water supplied will be progressively deprived of the ammonia and manganese content and biodegradable organic compounds via successive biological oxidisation stages within the layer of quartzite until obtaining complete removal of said chemical substances by the specialised bacteria which will be selected at different depths of the biological filtering bed. The graph on the right shows, as an example, a "possible" amino nitrogen elimination profile according to the height of the biological filter bed. It can be assumed that the order of oxidisation of the substances present is: ammonia, manganese, organic compounds. The greater the concentration of the former, therefore, the less efficient the elimination of the latter.

For correct growth of the biological mass in the filter, a metering system for any micronutrients, necessary for biological growth, can be provided on the biological filter supply.

As the pressure losses through the quartzite bed increase, backwashing with water and air will be sufficient to restore correct operation of the biological filter. The plant is provided with a regeneration system and air/water backwashing which can be activated automatically.

 

The micro organisms, grouped in colonies, remain securely fixed to the bed which acts as a support where they live and multiply without passing into the treated water.
The microbic load is absent or insignificant and in any case well below the limit established for drinking water; furthermore the micro organisms of the biological filter are compatible with humans since they are widespread in the environment, and are not pathogenic.

Plant description and sizing criteria

The "Wabag" biological filter consists of a vertical receptacle containing a thick uniform granular layer of inert material (quartzite, river sand), crossed from top to bottom (and also from bottom to top) by the water to be treated, previously saturated with oxygen. The solid products of the cell metabolism are retained in the granular bed and subsequently expelled by periodic washing. The plant solution is relatively simple; in fact it consists in a first stage performed under pressure by pulverising the water to be treated in a closed receptacle (oxidiser) in a current of air or pure oxygen, or at atmospheric pressure, in a vertical column (aeration tower), in a counter flow with air.
The first solution, applicable when methane is absent or present in a limited quantity in the water, does not require subsequent pumping, while the second, which must be used in the case of a significant methane content, requires pumping; this solution, however, has the advantage of intense elimination of any volatile organic substances that may be present in the water.
The second stage consists of a vertical column (filter) under pressure, containing the inert granulate bed, back-washable with water and air, when the accumulation of insoluble residue causes clogging.

The thick layer filter is particularly suitable for treating well water and groundwater, for example removal of iron, methane, manganese, ammonia and, since it is back-washed with air and water, i.e. using a very vigorous washing system, it is ideal also for the treatment of water containing significant quantities of fine sand.

Washing with air and water is very vigorous, effective and rapid (15-25 minutes) and consumes very little water - in fact it is the large quantity of air at low pressure (specific air flow rate equal to 80 m3/m2 h at a pressure of 6000 m water column) which causes detachment of the materials retained by the filtering sand, while the water has the job of conveying them away. The uniform granulometry sand bed has a high capacity for retaining solids, which guarantees long filtering cycles even with very turbid water.

The filtering stage, according to the type and concentration of the compounds present, is sized with a filtering speed in the range 10-18 m/h, with granulate filtering layer thickness in the range 2000 - 3000 mm.

Plant start-up and process stability

The formation and above all growth of the biomass to a sufficient quantity requires a certain time according to the substances present to be eliminated: the longest start-up required is for elimination of the ammonia and manganese, while methane and hydrogen sulphide require much shorter times. For iron, start-up is practically immediate.
Once started, the biological filter operates instantaneously and is not sensitive to variations in the hydraulic load.

Advantages of the biological technique

Biological filtration has considerable advantages with respect to other systems, such as: oxidation by chemical-physical means (chlorine, chlorine dioxide, ozone, permanganate, UV rays), ionic exchange, adsorption:

a) it is a natural process: biological filters often occur naturally in the ground in favourable conditions;
b) as it is a natural process, once the conditions are established for maintaining it, absolute operating safety is guaranteed which is not always the case with the chemical-physical processes, mainly because the latter form sub-compounds which cannot be easily identified and eliminated, and their effects on the human organism are still not fully known;
c) running costs are insignificant and it is therefore ideal in a world geared to energy saving.

With respect to the oxidation technique by means of hypochlorite, biological filtration has the advantage of avoiding oxidation at break point of the ammonia which requires metering of an excess of chlorine with respect to the ammonia in the amount of 10/1.

The high level of chlorine, in addition to being costly, involves the risk of formation of organic chlorine compounds due to the presence of humic compounds and reduced quantities of methane in the well water. Regulation of this type of treatment is critical: insufficient chlorine does not guarantee complete treatment while an excessive quantity causes problems in the subsequent sections. In addition, as long as complete oxidation of the ammonia present is not guaranteed, complete oxidation of the manganese cannot be obtained.

The amount of chlorine dioxide replacing the hypochlorite does not guarantee the elimination of ammonia, a compound present at a concentration value very near to the potability limit value. In addition the amount of dioxide involves the risk of the presence of residual chlorite, a compound for which very restrictive water potability limits exist.

To avoid these problems, EUROTEC WTT proposes oxidation by air. This technology, compact and simple, is even cheaper both in terms of running and investment than the one using oxidation with chlorine which necessarily requires subsequent treatment on carbon safety filter.

References

As regards the validity of the technology proposed, it should be underlined that the water boards in the Po Valley are currently replacing the old chlorine oxidation plants with new biological filtration plants, a process that has been under way for several years now.

We remain at your full disposal should you wish to organise a reference visit to a potabilisation plant produced by Eurotec WTT using the proposed technology for treatment of water containing ammonia, methane, iron and manganese.

Eurotec WTT Srl has various pilot plants with quartzite filters of different sizes, available for performing pilot experimentation directly on the water source to be treated.