Industrial symbiosis reduces water
demand
Case study: Cemex, Colombia
Company Overview
CEMEX is a global building materials company that provides products and services to customers
throughout the Americas, Europe, Africa, the Middle East, and Asia. Cemex produce, distribute, and
sell cement, ready-mix concrete, aggregates, and related building materials in more than 50
countries.
In South America, Cemex is Colombia’s second largest cement producer with the capacity to produce
4.8 million tonnes of cement annually from its 4 cement plants. This cement is supplied to
Colombia’s construction sector and is used as a raw material for the production of concrete at their
25 ready mix concrete plants across the country.
Cemex currently holds a significant share of the ready-mix concrete market around Colombia’s three
largest cities (Bogotá, Medellin, and Cali) - accounting for approximately 44% of the country’s
cement consumption. Its Morata ready-mix plant, located in central Bogota, employs forty staff and
produces around 200,000 tonnes of ready-mix concrete per year.
Process overview
Ready-mix concrete is a building material that is produced in ‘batching’ plants and delivered directly
to the building site. It is produced from mixing cement, with fine and coarse aggregate, water and
admixtures (which control the properties of the concrete including plasticity, pumpability, freezethaw resistance, strength and setting time). By combining water with this cementitious material a
cement paste is formed through the process of hydration. The cement paste glues the aggregate
together, filling voids within it and allowing it to flow more freely. Less water in the cement paste
will yield a stronger, more durable concrete whereas more water will give a ‘freer-flowing’ concrete
with a higher ‘slump’ or workability’ rate.
The hardening of the concrete involves many different reactions, often occurring at the same time.
As the reactions proceed, the products of the cement hydration process gradually bond together the
individual sand and gravel particles and other components of the concrete, to form a solid mass.
Impurities in the water used to make concrete can cause problems when ‘setting’ or in causing
premature failure of the structure. The quantity of each of these components is crucial to the final
product and stringent controls are set in place during the manufacturing process to guarantee the
quality and consistency of the finished product.
The raw ingredients are stored in hoppers and silos and mixed using advanced computerised
systems. The process is controlled by experienced batchers who check every load to ensure it meets
the customer’s requirements (Figure 1).
Figure 1 Diagram of production process
Constraints on water use
The Morata plant uses approximately 34,800m3 of water per year of which the majority (up to 80%)
is used in concrete production as an integral part of the product with each tonne of ready-mix
concrete requiring around 180 litres of clean water. The remainder (~20%) is used for cleaning
machinery and on site dust suppression. The site operates a closed system which enables wash
water and surface run-off to be collected and treated by settlement before re-circulation into the
process.
Until 2011, apart from a small (10%) amount of rainwater and surface run-off collected on site,
water was primarily (90%) obtained through the municipal supply at a cost of 5,000 Colombian
Peso’s / m3 (~US$2.5 / m3). As such, the cost of water (900 Peso’s per tonne product) led Cemex to
consider alternative sources. However alternative supply options to municipal water were limited as:
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Groundwater permits are difficult to obtain as it is already heavily abstracted in urban areas.
Many of the rivers in urban areas are contaminated by untreated sewage, industrial and
mining effluent. In Bogota, approximately 20% of discharged sewage is treated in its only
wastewater treatment plant with the remainder discharged directly to rivers untreated.
Adjacent to the ready-mix plant is the Meals De Colombia ice cream factory which produces
approximately 84,000m3 of wastewater per year. Due to strict hygiene constraints, the factory is
unable to re-circulate its wash water within the process so following on-site treatment all
wastewater was discharged via the sewerage system to river.
Following discussions with Meals Cemex was able to establish the potential for an industrial
symbiosis in that with further treatment to remove the organic component the wastewater from the
ice cream factory could be used for concrete production and vehicle washing. Since both wastewater
discharge and water supply are relatively expensive in Bogota, there was motivation from both sides
to pursue the opportunity further.
One of the main constraints in the use of the wastewater for concrete production was the chloride
content. Salt is added to the wash water by the ice cream factory to lower the freezing point of the
water used for cleaning. As such the water has to be blended with municipal water to ensure the
chloride concentration remains below the limit required for concrete production. Initially the factory
reported on water quality to Cemex three times per day and based on this information, Cemex was
able to regulate its use of the water. However, continuous on line monitoring of chloride content in
the wastewater from the ice cream factory has now been installed which has allowed the process to
be managed more efficiently by the process managers from the two sites.
During the first few months the plant used around 10m3 / day equating to 13% of its requirement
but this has now increased and Cemex is able to obtain up to 30% of its requirement from this
source. At present this alternative supply is one fifth of the cost of municipal water. While sufficient
water is available from the ice cream factory it is unlikely that it will ever be cost-effective to reduce
or remove the chloride content such that it totally meets Cemex’s water requirement. As such
around 70% of Cemex’s water will still be obtained from the municipal supply.