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Home Biofilters - Biological filters to remove greywater nutrients
Domestic Biofilters in practice

A biofilter is designed to treat greywater by biofiltration, which combines mechanical retention through filtering material and biological transformation of the pollutants in the water to be treated, eliminating a significant amount of pollutants before they reach the groundwater, river or natural wetland. The system can be designed for a single house or groups of houses. The size of the system varies according to the volume of water treated.

Greywater contains nitrates, phosphates, soap, salt, bacteria, foams, food particles, organic matter, suspended solids, perfumes and dyes. Greywater originates from households, schools and all places where water is used for cleaning purposes, excluding excreta. It is the product of laundries, bathrooms, sinks and other household uses. Home Biofilters are a sustainable way of removing pollutants from greywater. Below are details of the operations and processes involved in biofilter water treatment.


Primary treatment or pre-treatment

Pre-treatment takes place before the biofilter comes into operation, and involves separating the free floating material and sedimentable matarial. The containers are usually plastic tanks, used to separate fat, their size depending on the volume of greywater to be treated. One or more containers can be used to improve removal. Sedimentable materials sink to the bottom of the tank due to differences in specific gravities. In this way fats and detergents are separated and free floating material comes to the top of the tank.

The systems promoted usually have two fat separator tanks to ensure removal of as much material as possible. This step is essential if the system is to function properly, since it prevents larger particles from reaching the biofilter. Routine maintenance for this process involves the removal of solids and fat stuck to the tanks. Maintenance frequency depends on the volumes of water discharged and the number of solids present; however, it should be done at least once or twice a week.


Biological treatment (biofilters)

A biofilter consists of a trench, dug by hand and waterproofed walls, with a filter bed made of gravel, volcanic rock or other material having similar characteristics. Within the substrate a biological process takes place triggered by bacteria that are found naturally within it and in the roots of plants, which break down nutrients in both the water and the soil.

The residual water is in contact with aerobic and anaerobic zones, usually for a few days, giving rise to microbiological and physicochemical degradation processes. Aerobic zones are provided by plant roots, injecting oxygen by natural processes; the anaerobic zones are those away from the roots of plants and which generate conditions conducive to the development of bacterial film. Biofilter maintenance takes place sporadically, depending on the frequency of fat separator tank maintenance or pre-treatment and involves washing the filtering material found in the biofilter’s first two meters of length.

The filter bed provides a base for growing water plants or microphytes. Because home biofilters are located close to houses, we recommend sowing ornamental plants. Recommended plants for this type of system include:


Water use (disposal or utilization)

A decision has to be made on the final use of the water treated in the biofilter system. Since this water is clear, not turbid and free of much of the original organic material, it can be reused for irrigation, washing or cleaning. This water can also be used to recharge groundwater through infiltration. Biofilters have been proved to remove the following:

Parameter analysed	Pollutant removal percentage (%)
Fats and oils	92-96
BOD (mg / L)	85-95
COD5 (Mg/L)	95-95
NO3 (mg/L)	90
Total Phosphates	85
S. Solids  Total (mg/L)	90
Sedimentable solids (mg/L)	95
SAAM (mg/L)	80
Fats and Oils	93

Source: S. Chávez y O. Guevara, Evaluación del sistema de tratamiento de aguas residuales domiciliares por biofiltros de flujo horizontal, del programa ISSUE 2 en barrios del distrito V de Managua, 2010


Stages in the implementation of a Biofilter

Design of a home biofilter

To calculate the size of a home biofilter, you need to consider:
• The number of people living in the houses
• The estimated amount of water that these people use.

Therefore, size will be based on the average amount of wastewater produced in the homes, which depends on the number of people for whom it is designed times the amount of water per person. To establish the quantity of water used by people, reference can usually be made to data from a country’s national water and sewage board.


Size of pre-treatment tank

For pre-treatment to function properly, certain conditions must be taken into account in the design phase, such as:
• Hydraulic retention time for proper sedimentation or settling of particles.
• Retention time for biodigestion.
• Space necessary for the accumulation of matter (defined by three volumes: liquid volume, sludge volume and fat volume).


Size of Biofilter

To decide on the size of the biofilter, the minimum width of the biofilter (B min) and the minimum length of the biofilter (L min) need to be calculated. The width depends on the depth of the biofilter, the slope and hydraulic conductivity of the filtering material. The length depends on the hydraulic load and amount of greywater. The calculation of the minimum length can also be used to decide on the ideal distance for construction.


Validation

Having made a preliminary decision about width and length, it validated by using a series of parameters that have to meet certain technical criteria:
• If the system is smaller that the actual requirements, efficiency will be lower. Experience has shown that the hydraulic retention time for a correctly sized system must be between 3 and 5 days.
• An empirical estimate must be made of the Biological Oxygen Demand 5 (BOD 5) to have an idea of the quality of water in the effluent. Similarly an estimate is made of the organic load and fecal coliform concentration in the effluent. Since the characteristics of domestic water are generally consistent, data can be used from studies and trials carried out in different regions.


Construction stages

The stages in the construction of a biofilter are as follows: place stakes and mark reference levels; excavate hole and level out the bottom; take and verify measurements; place plastic and sacks; prepare PVC pipes; place materials

After a pilot biofilter has been built, it is checked and monitored to make sure the system is working efficiently, taking influent and effluent water samples to measure pollution parameters such as BOD (Biological Oxygen Demand), nitrates and phosphates.

The efficiency of these systems depends to a large extent on maintenance, which is carried out at different stages. The process of monitoring and taking samples can be performed by certified laboratories or universities, which is recommended especially for pilot projects and initial studies of the systems.