Limited water resources is a reality being faced by Malta due to its status as a semi-arid country. Nonetheless, by means of sustainable water management there are several possibilities to ease the problem and avoid the overexploitation of this limited resource.
About 70,000 cubic metres of treated sewage effluent are being produced daily by four sewage treatment plants across Malta and Gozo. The acute groundwater overexploitation problems can be mitigated because treated sewage effluent is abundant for agricultural irrigation.
This effluent is being discharged in the sea but can be used after tertiary treatment. Water reuse is crucial for sustainable water management and, in this regard, there are several possible uses for treated effluents, one of which is agriculture.
Irrigation with treated effluent has been practised in several countries for many years but, unfortunately, in Malta many people have a wrong perception about its use. This is due to some farmers who used to irrigate with raw sewage to produce high yield products in a short period of time, which practice is unhealthy not to mention illegal. Another habit was heavy overwatering to increase the yield. However, such high water content increases hydrolysis and microbial growth rates, leading to a shorter shelf life of the crop.
Consequently, these practices triggered a negative perception about the potential benefits of effluent reuse. Yet, irrigation with treated effluents is an entirely different process.
The conductivity of irrigation water is the most crucial factor affecting crop growth. At the Sant’Antnin sewage treatment plant, treated wastewater is produced that is, at times, high in conductivity due to illegal high saline discharges into the public sewers. The saltier the water fed to plants, the less can be taken up by its roots due to the high osmotic pressure created at the root zone. This very often leads to the deformation of crops as they cannot take in the water they require for normal growth.
The Water Services Corporation is carrying out a pilot project in its sewage treatment plants to improve the suitability of effluents thereby increasing reuse potential.
Treated sewage effluent must be constantly monitored due to the requirements laid out in Directive 91/271/EEC. This monitoring ensures treated sewage effluent is of a good quality standard, meaning that by adding a tertiary treatment stage, good water for edible crops can be produced.
Tertiary treatment processes include reverse osmosis to reduce high salinity followed by ultra filtration to improve the microbiological quality of the effluent.
The study investigated the effects of different irrigation waters such as treated effluent, reverse osmosis water, groundwater and various blends. A total of 120 lettuces, Lactuca sativa L, were cultivated in a monitored greenhouse, using six different types of irrigation water of known quality. These different types of experimental waters included water from the Gozo and Sant’Antnin sewage treatment plants, groundwater and reverse osmosis water as well as their blends. This exercise produced some very interesting results.
Lettuce irrigated with treated effluent rendered significantly higher yields in terms of fresh weight and percentage dry matter. This is because treated effluents are richer in nutrients that may be lacking in soil. This proves that, without any fertiliser whatsoever and under equivalent conditions, the lettuce irrigated with treated effluent grew more efficiently. This is beneficial for both the producer and the consumer because the use of fertilizer can be avoided, lowering costs and helping the environment.
On the other hand, crops irrigated with high salinity water exhibited stunted growth due to the high osmotic pressure in the root zone.
Another finding was that lettuce grown with different waters had the same shelf life.
Excessive levels of nitrates in irrigation water are highly undesirable. The study showed that nitrate level in the lettuce’s sap was directly proportional to nitrates in the respective irrigation water. This is significant because, in Malta, there are areas where groundwater has high levels of nitrates due to farmers using too much fertilisers. Since groundwater is not monitored by farmers, there is no way of them knowing how much nitrate is present in the irrigation water they use. However, when the WSC eventually supplies treated wastewater to farmers this would always have been monitored prior to being supplied.
There was also a slight increase in the percentage of organic matter in soils irrigated with treated effluents. This is considered important because increased soil organic matter is beneficial as it is richer in nutrients for the crops, without the need of fertilizer. Moreover, organic matter increases the water retention capacity.
As the study showed, irrigating with treated sewage effluent renders benefits. Groundwater abstraction should be minimised and controlled and treated sewage effluent can be utilised, within the limitations and parameters of this resource in our island. However, further studies need to be carried out on other edible crops and landscaping projects before full-scale implementation.
In conclusion, the WSC has already taken the first steps to implement tertiary treatment. Another benefit of this strategy is to avoid overexploitation, which is resulting in seawater intrusion of the aquifer.
Diandra Caruana is a scientist working for the Water Services Corporation.