You are in: Home > Health and Environmental Hygiene - What's new > Bathing waters > Field tests of microbiological contamination of water in small bathing ponds

Health and Environmental Hygiene

Field tests of microbiological contamination of water in small bathing ponds

Last changed: 19/04/12

Small bathing ponds, as opposed to landscape pools, have become more and more popular among the public in recent years. There are at least 75 such sites known in Germany. They are man-made, outdoors, and capped underneath. A vegetated regeneration zone is designated for purification of the water, and unlike in swimming pool water, disinfection does not occur. Therefore, any pathogens possibly secreted by bathers are not immediately inactivated (as is the case in chlorinated swimming pools) but remain infectious for a prolonged period of time. Whereas pathogens in natural bathing waters are quickly rendered harmless to bathers thanks to dilution and natural decomposition processes, the bather-to-water volume in natural swimming pools (ponds) leads to a higher level of contamination than in landscape pools. Therefore pathogens in these ponds may be traceable for weeks. This causes considerable cause for concern with regard to the hygienic safety of such installations, especially in light of a publication about an outbreak of meningitis related to bathing in such ponds (meningitis outbreak of ECHO-Virus 30 in northern Hesse – Investigation of outbreak Source: Epidemiologisches Bulletin (2002), Nr. 20, 163-167, Robert-Koch-Institut).

A clarification of the purification capacity and development of criteria for safe operation are urgently needed.

The objective of one project conducted at the Federal Environment Agency is to make a step in this direction. It seeks to discover how effectively viruses and bacteria are eliminated by the vegetated regeneration zone. Other areas of investigation include

Deactivation of waterborne viruses by solar radiation,
Retention of viruses and bacteria in unvegetated sand filters and
Retention of viruses and bacteria in sand filters with reed vegetation

Experimental studies:

The project is carried out in part by using the Federal Environment Agency’s technical installation for the simulation of bank and slow sand filtration (SIMULAF). The experimental facility consists of a forebay, three sedimentation tanks, and a regenerative area. The forebay that simulates the bathing area has fresh surface water pumped into it at adjustable volume flow. The sedimentation tanks serve to separate out rough waste. The regenerative area has a bottom filtration system covered in planted reeds, composed of a gravel layer and integrated drainage as well as a filter unit of one metre’s thickness (see diagram). The following experiments have been carried out at this facility:

Accelerated tests: To simulate desired contamination in tests of short period contamination, suspensions of various (non-pathogenic) bacteria and viruses were produced and added to the “bathing area”. Water velocities in the filter (filter velocities) were 300cm/d (slow), 600cm/d, (medium) and 900cm/d (fast).

Long-term tests: These tests simulate long-term (2 weeks and longer) heavy contamination of the vegetated bottom filter. Municipal wastewater (as surrogate for pathogen input from bathers) was added up to a concentration of 1% wastewater in the bathing area. Slow (300cm/d) and fast (900cm/d) filter velocities were programmed.

The number of bacteria and viruses found in the bathing area and beneath the vegetated bottom filter were taken as criteria by which to evaluate purification capacity.

Photo of pilot plant (click to enlarge)

Diagram of experimental facility (click to enlarge)

The project is near completion. Interim results are as follows:

Viruses are rendered inactive in open bodies of water at a much slower rate than bacteria, a fact which must be taken into account during assessment and establishment of hygiene parameters.
In short-term pollution conditions bacteria and viruses were reduced by a factor of 10 to 1,000 by means of drainage through the vegetation-covered bottom filter. Purification performance is highest during slow filtration.
After periods of long operation, i.e. when the bottom filter has adapted and is grown-over, significantly higher levels of treatment are achieved. Greater filter velocities also result in considerably improved purification.
Elimination capacity varies among different types of viruses and bacteria.

Outlook: Pending questions as concerns a more comprehensive characterisation of elimination capacities of vegetated bottom filters are:

How does filter capacity change in a recirculation system?
Will filter capacity become impaired if bottom vegetation receives no nutrients over a longer period of time and therefore slows its growth?
How can water flow in the filter inlet be designed (e.g. on wide, very shallow terraces) to more effectively render pathogens inactive by means of solar radiation?
Which types of soil (clay?) are especially effective pathogen binders?

Furthermore, simulation experiments can only test a fraction of the conditions that exist in real life. Therefore the next step to augmenting these efforts would be to take up selective microbiological testing of the waters at visited bathing areas, especially at peak periods. These tests should be evaluated in relation to how the bathing area is operated, intensity of use, and if possible, any conspicuous incidences of illnesses. This might be the basis for development of guidelines for the operation of small bathing ponds that defines the number of visitors relative to water volume and biotechnical constitution.

Issues