Sanitization of water system

By / Ahmed Hasham

Water & Quality Expert


Definition of Sanitization

Sanitization is not an absolute process. It is a partial removal of organisms. The sanitization process should reduce the organism population by some 90%. In water, sanitization is often defined as a 3-logarithm (log) or 1,000-fold reduction in the number of bacteria.


Sanitization tactics:

There are two basic tactics for controlling bacterial growing in a potable water system:

  • The first tactic is to keep a constant residual level of biocide agent within the system (continuous dosing).

As example for this technique that water treatment facilities use when they inject sufficient chlorine to provide a residual throughout the distribution system.

  • The second tactic is to periodically sanitize. If for some aim the process protocol not allows the use of continuous chlorination, then periodic sanitization will be essential.

Chemical Sanitization:

Chemical biocides can be divided into two main groups:

  1. Oxidizing: contain chlorine, chlorine dioxide, and ozone.
  2. Non-oxidizing: contain Quaternary ammonium compounds, formaldehyde, and anionic and nonionic surface-active agents.

This table provides some general information about biocides. The table includes recommended contact times for various concentrations, as well as factors to consider when choosing a biocide to use with automated watering systems. Note that some biocides are not recommended for use with automated watering systems at all.


The most common sanitizing agent is chlorine. Chlorine is the cheapest, most readily available, and is effective and easy to handle. Even though ozone and chlorine dioxide are also effective biocides, there is little understanding using these chemicals to sanitize automated water systems. The effectiveness of a sanitizing chemical depending on both concentration and contact time.

Typical sanitization of an automated water system is accomplished using 20 ppm chlorine for 30–60 minutes. Higher concentrations or longer soak times will increase effectiveness; however, do not use a sanitizing solution with a chlorine concentration higher than 50 ppm. Repeated sanitization at higher concentrations can cause corrosion of stainless steel wetted components in an automated watering system.

Thermal Sanitization using Hot Water

Heated water may be used to sanitize a system if it is held in the range above 70°C (158°F). The practical characteristics of handling water at this temperature (the materials of construction and the energy used).

Sanitization Frequency

Sanitization does not kill 100% of bacteria in a watering system, the remaining bacteria can re grow in the system. This means that the components of a water system will need to be re sanitized periodically.

The frequency for your particular system will depend on its design, the frequency of both flushes and filter changes, the supply water quality, and the bacterial quality you are trying to maintain. To determine the sanitization frequency, establish a regular schedule for drawing samples and monitoring the total bacteria count levels. Increase or decrease the frequency of sanitization based on the measured bacterial quality. To destroy an established biofilm, (for example: a watering system that has been in operation for some time and has never been sanitized) repetitive sanitizing cycles are usually required.

The initial chlorine contact may only kill the top sheet of biofilm. Chlorine will also destroy the glycocalyx or slime which is the “glue” that holds biofilm bacteria composed and to the pipe wall, this weakens the biofilm structure. For that reason, it is a great idea to follow chlorine exposure with a high-flow flush. Fresh chlorine is then injected again to the piping to kill the next bacterial layer. This chlorine sanitization/flush cycle may need to be repeated more than a few times on successive days till the gathered biofilm has been removed. For a well-established biofilm, 3-10 cycles may be need.

Sanitization of an Automated Watering System

All the components in an automated watering system should be sanitized at regular intervals. This section describes how to sanitize these components.

RO Units

Continuous chlorination for feed water:

 For reverse osmosis (RO) systems using cellulose acetate membranes, continuous chlorine pretreatment is used to prevent bacteria growth in the RO machine. Chlorine injection is adjusted to provide 0.5 – 2.0 ppm of free chlorine in the feed water and a minimum of 0.3 ppm free chlorine.

Clean-in-place cycle for RO unit:

Regular cleaning of the RO machine is essential because contaminants can precipitate or scale on membrane surfaces, reducing flow rate and quality of the product water. On most of the RO machines, cleaning is done automatically on a periodic basis. Low pH cleaners (Such as citric acid) are used to remove precipitated salts and metals, and alkaline (Such as NaOH) or neutral cleaners are used to remove dirt, silt, and organic foulants.

RO membranes can also become fouled with microorganisms. To minimize biofouling, it is best if the RO machine can operate continuously, or as many hours a day as possible, to minimize stagnant downtime. If a microbiological cleaner is needed, follow the membrane manufacturer’s recommendations.


Joymalya bhattacharya, Sanitization of automated watering system, Generation of pharmaceutical water. CreateSpace Independent Publishing Platform, 2013. — 134 pages, ISBN: 1492393495.

Aerobic Treatment with Biofilm Systems

Aerobic Treatment with Biofilm Systems


         Biofilms are small ecosystems usually consisting of three layers of differing thickness,

which change in thickness and composition with location and over time (Meyer-Reil 1996). In the first phase of colonization, macromolecules are adsorbed  at clean solid surfaces (proteins, polysaccharides, lignin; Wingender and Flemming 1999), because they are transported from the bulk liquid to the solid surface faster than the microorganisms are. As a consequence of this adsorption, the coverage of the solid surface with water is reduced. During the second phase, microbial cells attach to this prepared surface. Frequently, they do not form closed layers of uniform thickness, rather they form small attached colonies, which may spread by growth and further attachment. Usually, these cells are supplied with substrate and oxygen and are able to grow at their maximum rate. During this process, they produce organic molecules, which diffuse through the cell wall and to extracellular polymeric substances (EPS) catalyzed by exoenzymes. These EPS molecules are necessary for the formation of a stable biofilm (Wingener and Flemming 1999). In the third phase, the biofilm may consist of bacteria and EPS, the thickness of which is a function of growth rate and depends on the stability of the biofilm and the shear stress of the flowing water (Van Loodsrecht et al. 1995). At lower shear stresses ,eukaryotic organisms (protozoa, insects, their eggs and larvae) typically establish themselves. All these organisms live in a community. Materials such as substrates and oxygen are transported into the biofilm by diffusion and convection and the products are transported out of the biofilm.

          Oxygen may reach only into the exterior part of the biofilm, resulting in a growth of aerobic microorganisms such as nitrifying bacteria and protozoa. Nitrate and nitrite produced in this layer are reduced by anoxic metabolism within a middle layer, resulting in an anaerobic interior layer directly at the solid surface, where acetic acid and sulfate may be reduced (Marshall and Blainey 1991; Fig.1).

Heterogeneous biofilms grow on the sides of ships and on buildings near the water’s edge, inside human and animal mouths and within inner organs. They frequently cause damage to these surfaces (biocorrosion) and must be removed. In the area of environmental biotechnology, however, they can be utilized to advantage in certain bioreactors, such as:

  • trickling filters,
  • submerged, aerated fixed bed reactors,
  • rotating disc reactors.

The formation of biofilms is a requirement for their effectiveness.

Fig.1 Biofilm model (according to Marshall and Blainey 1991).

Trickling Filters

        A trickling filter consists of a layer of solid particles or bundles of synthetic material inside a cylindrical (Fig..2) or prismoid container. Wastewater must be distributed uniformly at the top of the fixed bed – frequently by a rotating system of two or four horizontal tubes equipped with many nozzles.

To compensate for the fact that the area of a circular section of the reactor increases  with distance from the center, the distance between nozzles must decrease the further they are away from the center in order to have an even distribution of water over the surface. Furthermore, the changes in available pressure in the rotating tubes must be considered as a function of the flow rate. Uniform distribution of the wastewater and uniform packing of the reactor with solid substances are of high importance for a high loading and removal rate. It is critical to ensure that two conditions are met:

The downward flowing liquid films must be in direct contact with the biofilm (i.e. the biofilm has to be trickled over all places and at all times) and must be in contact with the upward or downward flowing air (i.e. the trickling filter should not be flooded at any location or time).

              The wastewater must be practically free of solids. It is absolutely necessary that the  wastewater passes a primary settler under controlled conditions which is never overloaded.

We distinguish between:

Natural aeration as a result of density differences between the air saturated with moisture inside the trickling filter and the air outside the trickling filter, and

Forced aeration by a ventilator at the top of the trickling filter. In this case, the reactor may have a height of up to 12 m and is filled with packages of synthetic supporting material.

Fig. 2 Trickling filter, BIO-NET, Norddeutsche

Submerged and Aerated Fixed Bed Reactors

In cases of high hydraulic loading, the trickling filter may be operated as a flooded bed and the pressure differential needed for the downwards flow increases. The level of wastewater necessary to overcome the flow resistance depends on the form of the substance used as support material and the thickness of the biofilm. Aerobic fixed beds must be aerated near the bottom, producing a two-phase flow in a three phase system with an upwards air flow. As a result of friction forces, water is transported upwards in the center of the reactor and flows downwards near its walls.

Biomass is attached at the surface of the support material and is also suspended as flocs. It is not easy to avoid blockages in regions of biofilms with a higher thickness and a lower local flow rate. The fixed bed must be cleaned from time to time by considerably increasing the wastewater flow rate.

Synthetic support materials such as BIOPAC (ENVICON, Germany) have been used successfully, especially where nitrifying bacteria with lower growth rates must be immobilized (Fig..3).

In contrast to fixed beds with solid particles, the flow of water and air are more easily controlled and blockages can be avoided in reactors with suspended particles.

In contrast to trickling filters, their air flow rates can be adjusted to match the loading of organics and ammonia. The specific surface area can be increased to up to 400 m2 m–3 (Schulz and Menningmann 1999). Using membrane-type tubular aerators, fine bubbles are produced and the mass transfer rate is increased remarkably.

The suspended biological sludge detaches from the surfaces as a result of the friction forces of the flow and is   conveyed to the secondary settler. Obviously, blockages do not occur.

Fig. 3 Submerged aerated fixed bed reactor

(a) and BIOPAC (b)(ENVIRON, Germany; Schulz and Menningmann 1999).

Rotating Disc Reactors

In rotating disc reactors (RDR), the principle behind the intense transport of substrates and oxygen to the biofilm is different. In trickling filters and fixed bed reactors, water and air are moved; here, the support material with the biofilm are moved. In rotating disc reactors, circular plates with diameters of 1–2 m are fitted to a horizontal shaft with a spacing of a few centimeters. The system of parallel plates is submerged nearly halfway in a cylindrical tank through which wastewater flows. The packet of plates rotates at a speed of 0.5–5.0 rpm. Bacteria grow on both surfaces of the circular discs. During the portion of the rotation where the biofilm travels through the air, wastewater drips down and oxygen is taken up by convection and diffusion. Parts of the biofilm rinse off from the discs from time to time.

Larger pieces settle in the tank and must be removed as surplus sludge, while smaller parts are suspended and involved in aerobic substrate degradation and further growth (carbon removal and nitrification).


Ahmed Ahmed Elserwy

Water & Environmental Consultant

Ain Shames University, Faculty of Science


  • Marshall, K.C.; Blainey, B. 1991, Role of bacterial adhesion in biofilm formation and bio corrosion, in: Biofouling and Biocorrosion in Industrial Water Systems, ed. Flemming, H.-C.; Geesey, G.G., Springer-Verlag, Heidelberg, p. 8–45.
  • Metcalf, Eddy 1991, Wastewater Engineering: Treatment, Disposal, And Reuse, 3rd edn, McGraw-Hill, New York.
  • Meyer-Reil, L.-A. 1996, ضkologie mikrobieller Biofilme, in: ضkologie der Abwasserorganismen, ed. Lemmer, H.; Griebe, T.; Flemming, H.-C., Springer-Verlag, Berlin, p. 24–42.
  • Wingender, J.; Flemming, H.-C. 1999, Autoaggregation of microorganisms: flocs and biofilms, in: Environmental Processes I, (Biotechnology, Vol. 11a), ed. J. Winter, Wiley-VCH, Weinheim, p. 65–83.