From Mr M Causon
A new sewage treatment plant (STP) is to be installed at Ic-Cumnija, Mellieha. This will bring Malta in line with its European counterparts and in compliance with the laws for the disposal of sewage waste. It has been the norm for Malta to eliminate all its sewage by flushing it into the sea.
Alternattiva Demokratika has been bringing this issue to the forefront of its environmental and health awareness campaign for years on end and this new plant that will eliminate such a bizarre way of dumping our sewage will be welcomed by all.
AD believes that if the plant incorporates a system of using anaerobic digestion in a biotank, this will not only eliminate our sewage waste but will also be beneficial to our economy.
The environmental statement envisages that it will have a conductivity of 5,600 microsiemens per cm while, according to the FAO, the maximum allowable level is 2,000. Maltese farmers usually tolerate up to 4,000. The STP water salinity could be reduced to 4,500 by a number of long-term measures (mainly, reducing the salinity of tap water and stopping hotels connected to the government sewerage network using sea water for flushing). However, 5,600 or, at best, even 4,500 is still higher than many of the boreholes in the area of Pwales, etc.
If this were to be done, it would destroy the underlying aquifer forever! The water produced by the Cumnija STP will – very likely – be good for nothing.
Why do we say this? Anaerobic digestion is a complex biochemical reaction carried out in a number of steps by several types of microorganisms that require little or no oxygen to live. During the process a gas, principally composed of methane (CH4) and carbon dioxide(CO2), otherwise known as biogas, is produced. The amount of gas produced varies with the amount of organic waste fed to the digester and, in addition, temperature influences the rate of decomposition (and gas production).
Anaerobic digestion occurs in four distinct steps:
Hydrolysis: whereby complex organic matter is decomposed into simple soluble organic molecules using water to split the chemical bonds between the substances.
Fermentation or acidogenesis: the chemical decomposition of carbohydrates by enzymes, bacteria, yeasts, or molds in the absence of oxygen.
Acetogenesis: whereby the fermentation products are converted into acetate, hydrogen and carbon dioxide by so-called acetogenic bacteria.
Methanogenesis: whereby methane (CH4) is formed from acetate and hydrogen/carbon dioxide by methanogenic bacteria. The acetogenic bacteria grow in close association with the methanogenic bacteria during the fourth stage of the process. The reason for this is that the conversion of the fermentation products by the acetogens is thermodynamically only possible if the hydrogen concentration is kept sufficiently low. This requires a close symbiotic relationship between both classes of bacteria. The anaerobic process only takes place under strict anaerobic conditions (ie the absence of oxygen and very low redox potential). It requires specific adapted biosolids and particular process conditions, which differ considerably from those needed for aerobic treatment.
The advantages of anaerobic wastewater treatment are as follows:
After completion of the anaerobic digestion steps, the pollutants in the wastewater are transformed into methane (CH4), carbon dioxide (CO2) and a small amount of biosolids. Since the solubility of methane in water is very low, it escapes as methane gas.
As a result, a significant part of the energy originating from the pollutants leaves the system as biogas, leaving only a fraction of the initial energy for assimilation by the biomass. Hence the amount of energy generated by this pathway, which can be used by the biomass, is only a small fraction of the total energy content of the incoming pollutants.
Biomass growth is, therefore, much lower than the growth prevailing in the aerobic processes.
For that reason, anaerobic treatment produces between five and 10 times less biosolids (sludge) than aerobic processes. In addition, anaerobic biosolids have the advantage of being much more compact than aerobic biosolids. The dry solids content of anaerobic biosolids range from two per cent (for a digester) to more than eight per cent for an up-flow anaerobic biosolids blanket process. Furthermore, anaerobic biosolids have much better de-watering characteristics compared to aerobic biosolids. As a result, the volume of de-watered biosolids from an anaerobic treatment is seven to 12 times lower than the aerobic processes.
Another advantage is that anaerobic digestion does not require energy-consuming aeration equipment. The production of biogas during the process even results in a positive energy balance. For this reason the biotank is designed to process the effluent by first using anaerobic digestion and then aerobic digestion.
...and its drawbacks:
The main drawback is that anaerobic digestion requires more stringent process control and only reduces the organic pollution by 85 to 90 per cent, which means a second step is usually needed to guarantee high effluent quality. This is usually an aerobic stage for polishing before discharge. As anaerobic biosolids production is rather low, the nutrient removal (nitrogen and phosphorus) is equally low as well. This is one more reason for applying an aerobic second stage which removes the residual nutrients, which would otherwise cause eutrophication of the sea in which the treated effluent is discharged.
Mark Causon
Environment spokesman
Alternattiva Demokratika