Position paper:
Reservoirs with an accelerated release mechanism – an immediately applicable engineering solution for reducing the damage of effluent discharge
[Updated May 2022]
Dr. Tom Topaz, Tal Sade, Merav Gilboa, Hadar Tzadka, Dr. Yair Suari and Prof. Gitai Yahel
The Israeli Center for Estuarine Research
Abstract
Continuous discharge of treated wastewater (effluent) into streams, even when it meets the Inbar standard for stream discharge, causes chronic oxygen deficiency in the water column and impairs the streams’ function as ecological systems and as recreation and leisure sites. When the effluent is of low treatment quality, such discharges also cause severe environmental nuisances and pose a risk to public health. Rehabilitating the streams requires a conceptual shift. To date, the only proposal that would enable the necessary change is the construction of dedicated reservoirs that would allow rapid release of surplus effluent into streams over a period of a few days, and only in cases where no alternative solution for using or treating the water can be found. It should be emphasized that this solution can be implemented almost immediately and with a relatively low investment for most coastal streams.
The Problem
The importance of streams to the natural environment, and as recreation and leisure sites for human society, is well known. Nevertheless, most of Israel’s coastal streams do not realize their environmental potential, since most streams in Israel suffer from pollution caused mainly by effluent discharges resulting from surplus accumulation and/or various malfunctions. Forecasts show that even larger surpluses of effluent will accumulate in the future, whether due to an increase in desalination capacity or as a result of changes in irrigation water standards.
Although the concentrations of nutrients and dissolved organic matter in effluent in Israel generally meet standards and regulations, discharging them into Israel’s coastal streams turns these systems into dead, oxygen-depleted zones even when the effluent meets the Inbar standard for streams (Figure 1). Most coastal streams end in a segment where water flow is slow and the stream and sea waters meet (an estuary). These areas extend over many kilometers and are used for intensive recreational activity such as parks and nature reserves. Recent studies show that water residence times in the estuary are relatively long, and therefore these stream segments are even more sensitive to nutrient and organic matter loads that cause oxygen deficiency and the formation of dead zones. In addition, recent studies show that effluent flowing into the sea remains close to the shore, exposing bathers to pathogens and additional health risks for extended periods (Figure 2).
Streams in the Mediterranean climate are, by nature, highly dynamic systems with a very high capacity for self-rehabilitation. Therefore, accelerated and well-managed effluent discharges can turn the pollution and effluent stress from chronic to time-limited, thereby significantly reducing the damage to streams and the health risks associated with the pollution.
The Proposed Solution
The solution for rehabilitating and managing coastal streams, and streams in general, fundamentally depends on stopping effluent discharge into streams. This requires an operational mechanism that prevents continuous discharge of effluent into streams as a result of malfunctions or the accumulation of effluent surpluses in winter. Currently, the effluent management system is not equipped for these scenarios, and the only available measure is continuous release of effluent into the streams. It is therefore proposed that large operational reservoirs be established or allocated, with the capacity for emergency release of the stored volume within a few days (an accelerated discharge). The reservoir would provide management, dilution, and long-term storage capacity, and would convey effluent from the various treatment facilities in the system, receiving the lowest-quality water in the system. The accelerated discharge mechanism would allow the reservoir to be emptied into the stream at high flow rates at the most appropriate timing (for example, before a flood or a winter storm) and in coordination with all stakeholders in the system. This discharge enables a reduction in environmental damage together with high management flexibility, including early warning to the general public regarding the condition of the stream and the sea.
Example Scenario: The Alexander Stream
Discharges from the Palestinian side of the Alexander stream’s watershed constitute a complex, dynamic, and unpredictable wastewater source. Building or allocating an operational reservoir with an accelerated-release capability would provide a response to the complexity of managing the Palestinian Authority’s wastewater. Thus, in cases of surplus effluent accumulation, or in cases where the wastewater cannot be treated, it would be transferred directly to the operational reservoir. An accelerated discharge of the reservoir’s water into the stream over a period of 24-72 hours – carried out only when the water cannot otherwise be utilized, and after advance notice to all management stakeholders in the watershed – would enable the release of the effluent and the emptying of the reservoir without causing long-term damage to the Alexander stream and its estuary.
For example, one of the possible sites for the proposed operational reservoir is located near the Bahan stream, where a peak flow of 48 cubic meters per second was measured during a flood event. Assuming the volume emptied from the operational reservoir would be 1-3 million cubic meters, a flood-like discharge could be carried out at a rate of 10-20 cubic meters per second, emptying the reservoir within less than three days. In a scenario of controlled discharge during the summer, coinciding with a summer storm, the marine environment is expected to recover within a few days, with the Beit Yanai beach expected to reopen up to a week after the discharge. The estuary is expected to recover ecologically within a few weeks. A winter discharge (at a known flow rate and through adapted channels), which is the preferred scenario, should be carried out according to the forecast, a few days before an expected flood flow, but not concurrently with a flood, due to the risk of flooding in the Bahan streambed. In such a scenario, the system is expected to recover almost immediately.
Further Reading:
Suari, Y., Sish, L., Gafni, S., Amit, T., Gilboa, M., Brookovich, A., Yahel, G., 2017. Continuous oxygen stress in the estuary of the Alexander stream. Ecology and Environment 8, 44-52.
Astrachan, P., Cohen, P., Paikin, S., Rubin-Blum, M., 2021. Occurrence of multi-drug-resistant pathogenic bacteria in Sharon-region stream effluents. Ecology and Environment 12(2).
Suari, Y. et al. A long-term physical and biogeochemical database of a hyper-eutrophicated Mediterranean micro-estuary. Data in Brief 27, 104809 (2019)
Suari, Y. et al. Sandbar breaches control the biogeochemistry of a micro-estuary. Frontiers in Marine Science 6, 224 (2019)