Water treatment: Why is AOM hard to remove?

Researchers from the Institute of Hydrodynamics, CAS, have revealed that the cause of low removal efficiency of AOM (Algal Organic Matter) originating from the green algae and cyanobacteria is low-molecular substances mostly of non-protein character. The research results have been published in the renowned Journal of Environmental Sciences.

Due to eutrophication (enrichment of water with nutrients – mainly nitrogen and phosphorus from agriculture, waste waters, etc.), an increased growth of algae and cyanobacteria in surface water sources occurs every year. Apart from the cells of the mentioned microorganisms, water contains also organic substances which are produced by these microorganisms or are released into water after their death. These substances are overall termed Algal Organic Matter (AOM). It is a complex mixture of substances (proteins, peptides, polysaccharides, organic acids, etc.) with very different molecular weight and other properties.

Coagulation/flocculation – a basic technology for AOM removal

The principle of coagulation – frequently used technology for water treatment – is a formation of suspension (insoluble particles) from the undesired impurities (AOM in this case), which are usually present in a soluble form in water. A coagulation agent (coagulant – ferric or aluminium salt) is dosed into water, where it interacts with AOM and due to mixing, flocs are formed. These flocs can be then settled and filtered through the sand filters. The important parameters in the coagulation process are the coagulant dose and primarily pH value which influences the charge and form of AOM and coagulant and therefore also their interactions and finally the efficiency of coagulation as a whole.

The most problematic is the low-molecular non-protein part of AOM

Considering that AOM is a complex mixture of substances, it is impossible to characterize it as a whole. Therefore it must be separated using various techniques, e.g. into protein and non-protein part or the groups of substances with different molecular weights.

After the coagulation tests were performed at optimum coagulant dose and pH, it was found that “non-proteins” (from green alga Chlorella vulgaris) required higher coagulant doses and simultaneously had significantly lower removal efficiency (only 25 %) than AOM proteins (60-85 %). Moreover, proteins and “non-proteins” could be removed at wholly different pH values. When an aluminium coagulant was used, the optimum pH for protein removal was 5-6.5 (weakly acid region), but for “non-protein” removal it was 6.5-8 (neutral to weakly alkaline region).

Molecular weight (MW) of AOM also influences the coagulation efficiency significantly. It was proved that high-molecular “non-protein” substances (MW > 100 kDa) with a low charge were completely removed on coagulation, whereas low-molecular ones (MW < 3 kDa) remained in water.

Is there a solution?

It is evident that the various AOM components are removed by different mechanisms (at different pH) and with different efficiency. Therefore, their removal strategy should be carefully evaluated. One of the possible solutions is that coagulation of AOM might be approached as a two-stage process: coagulation at acidic pH values for removing charged AOM proteins and coagulation at neutral/alkaline pH values for removing neutral “non-proteins”. This two-stage process requires further investigation as well as evaluation of the expenses of its implementing. The low-MW AOM (both protein and “non-protein”) remaining in water after coagulation require other removal processes, such as membrane filtration, adsorption onto activated carbon or oxidative degradation.

Source: Paper in the ScienceDirect database

Algal/cyanobacterial bloom on a water reservoir

Sampling and preparation for experiments

Coagulation tests

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