Natural treatment of wastewater

Introduction Wastewater purification is becoming a more and more relevant problem due to the growth of wastewater collection systems, which brings more and more wastewater to treat. Technological progress allowed the realization of advanced wastewater treatment systems, which answer the very strict legislation imposed by EU (Directive n.271 of 21st May 1991). In Italy e.g. small communities cannot afford the costs of these advanced and specialized systems, which require a trained and qualified staff as well. For this reason the development of very simple and cost effective natural wastewater treatment systems is particularly interesting. These treatments are called extensive, since they require long times and wide surface areas to become effective. Natural wastewater treatment systems, analyzed in the following pages, are: phytodepuration, lagooning and storage in tanks.

Natural wastewater treatments features

The main features of natural wastewater treatments are:

-          Simplicity: plants design and construction are very simple. Even small building companies can build them and unqualified staff can carry out their maintenance operations.

-          Cost-effectiveness: plants require low building, labour and maintenance costs. They are much more convenient than the conventional (biological) wastewater plants during the operational phase, because they require almost no energetic consumption or waste treatment. Mechanical devices are not used in these treatments, thus reducing the maintenance costs. The only limiting factor is the availability and the cost of land to place the treatment plants.

-          Efficiency: natural wastewater treatment plants are generally rather efficient for the removal of most of the pollutants. The efficiency is highly dependent on climatic conditions: it is lower with low temperatures.

-          Reliability: natural systems are very reliable even in extreme operating conditions. They can adsorb a wide variety of hydraulic and organic feed.

Extensive natural wastewater treatment systems are suitable to the depuration of waste produced by small or medium communities or by communities with variable population (touristy areas).

Potentiality

Natural wastewater treatment systems are used all over the world for the purification of wastewater from industry, household and agriculture. In Italy the potential is high due to the presence of several small or medium communities, the favourable climatic conditions, the availability of free lands, the growing interest towards treated water and the new limits imposed by EU Directives.
In spite of this, natural wastewater plants are still little diffused: there are about 200 plants all over the country. In many cases they would be a valid solution, because many communities cannot afford the expenses deriving from the use of conventional plants (13% of the existing intensive plants are not in operation).
Very interesting from an application point of view is the combination of extensive and intensive (conventional) wastewater treatments. It is important to use natural wastewater treatment systems considering the reuse of treated water. This is needed above all in arid regions.
Recently it is more and more preferred to treat wastewater deriving from households directly near the source (decentralized systems). This allows the use of natural wastewater treatment systems that are cheap and require low technological level.

Normative aspects

In Italy the EU Directive 271/1991 about water quality was transformed into the Legislative Decree 152/1999. Tables containing emissions limits for water streams, which flow to superficial water body, are contained in it. The limits are depending on the number of inhabitants and the type of area. The decree states the deadline before which local systems should answer the limits and it advises the use of lagooning and phytodepuration for towns having a population between 50 and 2000 people. The treated water can be used for irrigation, civil (street washing, cooling or hating plants feeding) or industrial purposes (process water). The decree states the minimum requirement for some chemical-physical and microbiological parameters. 

Conclusions

Natural wastewater treatment systems are simple, cost-effective and efficient methods to purify the growing amount of wastewater produced by our society. They can be applied as secondary or tertiary purification treatment, allowing the removal of most of the bacteria, microorganism and the destruction of the organic matter. Among them phytodepuration, lagoon purification and storage in tanks gave good results in terms of yield and are quite diffused all over the world.            
Their extreme simplicity in building, operation and maintenance make these systems competitive with the conventional (sewer) wastewater treatment methods.

Related topic:

Irrigation water quality