#insights

The biological treatment of urban wastewater reduces the organic matter present in the water and lowers the concentration of nutrients like nitrogen and phosphorus.

The first treatment stage in an urban wastewater treatment plant is always pre-treatment. This stage is essential for removing coarse and fine solids, sand and grease that can be entrained in the wastewater and reach the headworks of a wastewater treatment facility.

Before the biological treatment, primary decanting may be required, although it is not essential. In terms of efficiency, a primary decanting process typically removes 65% of suspended solids and 35% of the BOD₅ in the influent.

Biological processes are classified by the type of element to be removed or transformed, and involve the removal of carbonaceous organic matter or BOD_(C), the nitrification or removal of BOD_(N), denitrification and the removal of phosphorus. Biological processes are classified into aerobic, anoxic and anaerobic treatments, depending on the oxidation-reduction potential of the medium. Finally, depending on the way the biomass is located inside the reactor, it is classified as suspended culture or fixed or supported bed culture.

The biological process of activated sludge relies on a suspended culture. The reactor and secondary decanter work together, and by controlling the recirculation of sludge, the biomass concentration in the reactor can be maintained. Oxygen must be provided to the reactor, either by membrane diffusers or by means of mechanical surface aeration methods.

Other processes, such as percolating filters, biodiscs or rotating biological contactors, fixed bed IFAS systems and mobile bed MBBR systems, rely on fixed or supported bed cultures. In these systems, the biomass responsible for water purification is grown on media, usually plastics. Reactors work with a higher biomass concentration than a suspended culture, so they can be suitable for treating high pollutant loads. They are usually more stable in response to pollution spikes, and they can also reduce the space needed to implement the biological treatment.

In addition to the types of biological treatment analysed above, biological treatments with membranes, or MBR, are available. These systems incorporate ultra-filtration membranes into the biological process that separate the solid or sludge from the treated water. They allow working with high sludge concentrations, even higher than those of a fixed bed, thus reducing the volume required for the biological reactor. One of the keys to this treatment system is that the reactor and secondary decanter combination is replaced with a smaller reactor with ultra-filtration membranes, and the secondary decanter is not required to separate the water from the sludge. In addition, the quality of the biological reactor effluent is equivalent to that of a tertiary treatment, with the result being ultra-filtered water from which colloidal particles have been removed.

The removal of BOD₅ in a biological process of activated sludge, without reducing nutrients, is 93-95% efficient. The efficiency can be a little lower in biological fixed-bed processes, such as percolators or biodiscs, where it ranges from 60-90%. An MBR treatment provides an additional 99% efficiency in the removal of suspended solids.

MBR treatments also have certain disadvantages, such as increased investment costs mainly due to the price of membranes, which are proprietary technologies; and higher operating costs due to increased energy consumption and replacement costs, mainly due to the useful life of membranes (around 8-10 years).

In the water treatment projects we carry out at Sener Water Technology, we propose what type of biological treatment to implement in a treatment plant. This requires conducting a thorough analysis of the alternatives that weighs all the factors that may be decisive in the selection of the treatment. The factors to consider include the effluent quality targets, the sludge stabilisation level required, the versatility of the treatment, the climate, sludge production, the complexity of the operation and maintenance, the useful life, environmental criteria and, finally, economic criteria that take into account the investment, operating and maintenance costs.