Black and Grey Water: Closing the Loop Instead of Hiding the Problem

By Reyna Oleas & Roberto Plaza — Biologists and Co-founders of Montemar

When Water Disappears, Responsibility Often Does Too

In cities, we rarely think about what happens to water once it disappears down the drain. We take a shower, do the laundry, flush the toilet, and assume that someone—somewhere—will take care of the rest. Grey water and black water simply go away.

Globally, an average person uses between 100 and 150 liters of water per day for domestic activities such as showering, cooking, and cleaning. In many developed countries, that number exceeds 200 liters per person per day. The United Nations estimates that 50–100 liters per person per day are required to meet basic domestic needs under safe conditions. In most households, approximately 80% of that water returns to the system as wastewater, in the form of grey and black water.

That is how most of us grew up thinking. And for a long time, that was also our assumption here.

Until we understood that in the Galápagos, this logic does not work.

When the Problem Stops Being Invisible

In the Galápagos, the consequences of not thinking about wastewater are immediate and tangible. This challenge is common in island systems, where groundwater is shallow and contamination spreads quickly.

Here, we learned that untreated grey water was causing cross-contamination with freshwater wells, contributing to a high incidence of skin and gastrointestinal illnesses. The impact was not abstract. It was visible, measurable, and affecting daily life.

This realization forced us to stop and rethink everything.

We are located in the highlands, where the most common solution is the use of septic tanks. But when septic systems are poorly designed or improperly built, they can leak and contaminate groundwater. That is exactly what has been happening in many cases.

We decided not to replicate that model.

Learning From Nature to Design an Alternative

As we searched for a different solution, we turned to nature for guidance.

In natural ecosystems, the waste of one species does not become a problem—it becomes a resource for another. Nothing is wasted; everything is transformed. That principle became the foundation of our approach.

This is how Montemar’s on-site biological wastewater treatment system—based on biodigestors—was developed.

The system consists of four chambers built with lava rocks of different sizes, functioning as a biological filter. Grey and black water pass slowly through this system, where communities of microorganisms break down organic matter.

By the time the water exits the biodigestor, it has been treated and can be used to irrigate native plants. The result is a closed-loop system that treats wastewater on site, significantly reducing the risk of groundwater contamination.

Waste becomes part of the cycle again.

Treating Water On Site Changes Your Relationship With the Land

Treating wastewater where it is generated creates a very different level of responsibility.

Suddenly, water no longer disappears.

That shift led us to an unavoidable question: what are we putting into that water?

In cities, we rarely think about the soap used in the shower or the detergent placed in the washing machine. It does not seem to matter, because the water leaves the system. Here, it does.

Sooner or later, that water returns to the land—and to life.

Learning to Choose Products Compatible With Life

This system forced us to reexamine deeply ingrained habits.

Take laundry detergent, for example.

For most of our lives, detergent was something we used without question. This process pushed us to read ingredient lists for the first time. What we discovered was sobering: most commercial detergents are not compatible with living systems. They are designed to clean, not to coexist.

The same is true for chlorine.

Chlorine is a highly destructive substance that eliminates all forms of life. Introducing it into a biological treatment system would destroy the microorganisms that make the entire process possible.

We made a clear decision: not to use chlorine or chemical products that could damage the living system within the biodigestors.

This commitment led to a long and sometimes challenging search for products—detergents, soaps, and cleaning solutions—that could clean effectively without killing the microorganisms sustaining the system.

It has been, and continues to be, a meaningful learning journey.

A Living System That Demands Coherence

The biodigestor system operates every day. Quietly. Mostly invisible. Yet absolutely essential to the health of the land.

It also reinforces a fundamental lesson:

Sustainability is not only about infrastructure. It is about daily coherence.

Treating grey and black water on site taught us that every everyday decision—what we use to wash, clean, or disinfect—has real consequences. Living in a fragile ecosystem means acknowledging those consequences instead of outsourcing them.

When guests stay at Montemar, we are always willing to share the adaptations this journey has inspired.

Because closing the water cycle does more than protect the land. It transforms the way we inhabit it.

Sources

• United Nations. Water and sanitation – basic water needs. UN Water.
• World Health Organization (WHO). Domestic water quantity, service level and health.
• OECD & World Bank reports on domestic water use and wastewater return rates.

Figures cited are global averages; actual consumption and wastewater volumes vary by country, infrastructure, and household behavior.