Different territories, different solutions

Membrane technologies are essential tools in water purification and reclamation. However, the use of these technologies poses an often invisible challenge: management of the waste stream. Understanding how the waste is generated, what its characteristics are and how its environmental impact can be minimised are key to moving towards more efficient, coherent and sustainable sanitation systems, especially in coastal municipalities.

AIGUANEIX is a project that employs six different pro cesses to purify water. In each of these processes, the water – which originates from wastewater – improves in quality until it is pure enough to be returned to the aquifers. This means, however, that all of the com pounds extracted from the water must be treated as waste: the pollutants do not disappear. To this end, the source water is separated into two streams: the treat ed water (known as permeate) and the waste stream (known as concentrate). As the treatment becomes more rigorous, the quality of the water produced in creases; but so does the concentration of the sub stances retained in the waste material.

This effect is particularly relevant in the case of re verse osmosis. Although the technology is even able to retain ions of a low molecular weight, this results in a high concentration of salts in the waste material. In seawater desalination, with yields of around 50%, the result is a waste stream with a salinity approximately double that of seawater. For this reason, we must take great care when choosing the location of the facilities and the strategies for returning the waste material to the environment, in order to minimise its environmental impact.

Salinity and biological processes: two key elements

In wastewater purification processes, the components that need to be removed are very diverse: organic matter, nutrients, emerging pollutants, pathogenic microorganisms and salts accumulated during urban use and transportation through sanitation networks.

We must remember that the salt content of treated or reclaimed water is usually about 20 times lower than that of seawater. As the salinity of the waste stream generated is clearly lower than that of the sea, under appropriate conditions it is not expected to have a significant impact on the marine environment.

However, for this scenario to be truly viable, it is essential that the water that reaches the osmosis mem branes contains the minimum possible amount of organic matter. To achieve this, the performance of the biological reactors at the wastewater treatment plants (WWTPs) must be maximised. It should be not ed that each substance that can be removed through biological degradation represents a significant gain for the system as a whole: it improves overall efficiency, increases the reliability of the treatment and puts less demand on the more advanced stages.

If the secondary effluent is of good quality, it will not be necessary to correct parameters such as turbidity or ammonium concentration through the use of additional reverse osmosis processes, which are both energetically and financially costly. For this reason, sanitation facilities must operate in line with a holistic view of the entire system, from the sewer all the way to the final purification stage. The objective is to achieve the required water quality with the minimum consumption of resources, and to generate waste streams at the most appropriate points so that they can be managed correctly.

Managing waste in purification treatment

When each stage is performing at a high level, the waste from ultrafiltration chiefly consists of organic waste matter, which can be returned to the head of the WWTP. For its part, the waste from reverse osmosis is mainly composed of salts at concentrations much lower than that of seawater, which makes it possible to carry out controlled discharge through underwater outfalls in coastal locations. This approach, however, is not directly applicable to inland areas.

In such cases, reverse osmosis can generate waste with a concentration of salts that is too high for it to be discharged into the environment. However, since the areas in question do not suffer from saline intrusions into the sewer, it is possible to reduce or even dispense with this technology by using and bolstering other processes that are capable of removing emerging pollutants without concentrating salts. Examples of such processes include advanced oxidation treatments and filtration with activated carbon.

In coastal municipalities, therefore, wastewater purification represents the last opportunity to recover a resource as valuable as freshwater before it is irreversibly mixed with sea salt. With this aim in mind, it is necessary to focus on treatments that make it possible. It is difficult to generate new water resources in a more efficient, economical and sustainable way than by taking advantage of those that we already have at our disposal, right on our doorstep.

Everything that is removed from water treated by membranes is inevitably concentrated in the waste stream

Effective biological treatment is the best way to simplify and optimise advanced treatments

Water purification transforms a waste product into a strategic resource