Process Adaption of a Small WWTP Operated at Low Loading Conditions for Substantial Energy Savings Intermittent

The very long tradition of the activated sludge treatment model within the water industry has demonstrated very versatile possibilities to adopt the operation mode for different enhancements. By looking into other treatment models within the activated sludge family it is possible to find alternatives for the operation. This paper concentrates on the possibilities to improve even small WWTP with respect to energy savings. The small plant in Northern Sweden, called Rosvik WWTP, is given as an example. Some important findings related to the intermittent aeration mode may be summarized as follows: 1) An energy savings for the operation of the small WWTP with respect to aeration needs that resulted in a decrease of the energy power supply by more than 35%, as compared with the previous operation based on continuous aeration; 2) The up to date effluent levels with respect to the main pollutants have remained at very good levels in 2020, P-level averages 0.16 mg P/l versus consent level < 0.5 mg P/l; COD-level 40 versus < 70 mg/l and BOD 7 9 versus < 15 mg/l; 3) Sometimes, also improved sludge settling characteristics have been observed, thus providing improved discharge figures; 4) The potential to develop an enhanced biological phosphorus removal. There are however some needed conditions to accomplish these improvements: 1) Reliable on-line probes for both oxygen control, SS-concentration control and optionally also for nitrogen control; 2) A flexible automation system that allows the needed process modifications to take place; 3) And finally, very important dedicated and competent plant operators, with the needed curiosity for operation im-provements.


Introduction
The activated sludge concept for wastewater treatment is probably by far the most utilised system for organic removal, as well as biological nutrient removal ever since it was first developed by Arden and Lockett in 1915 [1]. The potential flexibility within the system has encouraged several different concept models to occur over a little more than a century. It should be borne in mind that the original configuration was as a matter of fact what we today known as the Sequencing Batch Reactor model. This goes back to the first scale laboratory tests performed in a very simple way, see Figure 1.
The development history of the activated sludge model is indeed a proof of the versatility of the concept. In this context, only a few cornerstones in the development would be mentioned.
• A first development step within the water industry was to take the fundamental concept "on board" but abandon the intermittent aeration mode and replace it by an aeration basin followed by a separate clarifier. • This model became by far the dominating concept for more than three decades, mainly due to a simple and easily understandable model. • In the 1950 is the development of the "Oxidation Ditch", especially for small WWTPs became a very important step. This is to be contributed to Dr. Passver, of the Netherlands. The impact of the Oxidation Ditch on a diversified activated sludge technology is illustrated in Figure 2.
• The further development that is still going on took on board the concept again of the intermittent aeration within the activated sludge system. Here only one of the many scientists that were instrumental is mentioned, see professor Robert Irvine [2].
In the following presentation the focus is made on small activated sludge plants in Sweden and starting with a historic background. In many respects it is correct to state that modern waste-water treatment started in Sweden in the late 1960s, when a new environmental law was implemented.
At that time, the environmental concern focused on phosphorus as being by  far the limiting factor for the Swedish water bodies. Once the environmental permits came into power the typical effluent stipulations were normally defined as follows: BOD 7 < 15 mg/l; Total P < 0.5 mg P/l. These permits were initially also given with specific, minimum percentage removal levels, typically > 90% both for BOD and P removal. This may be the starting point of a development of techniques to reduce especially phosphorus in the municipal wastewater discharge, see Morling, S. [3]. To successfully comply with these new standards a rather uniform technical solution developed during almost a decade in Sweden. For smaller plants, in this context defined as municipal WWTPs serving communities up to around 10,000 person equivalents. A very typical treatment scheme is illustrated in Figure 3.

Objectives
The objective of this study is to present the potentials to operate conventionally designed activated sludge plants intermittently. Examples are given from some plants where substantial energy savings are achieved by the intermittent operation.

Material and Methods
In the following, three different plants will be discussed, all operated by intermittent operation. Two of these plants have been presented previously, see [4] and [5]. One plant is a plant operating in Sri Lanka, labelled Ja-Ela WWTP (Wastewater Treatment Plant) and the second one is a plant in the center part of Sweden, labelled Solviken WWTP, serving the community of Mora, Dalecarlia.
The focus in this study is a small municipal WWTP facility located in the northern part of Sweden, own and operated by the public company PIREVA, owned by the community of Piteå. The plant is named Rosvik in the following.
The plant was built in the late 1970s, sized for 2500 person equivalents. The process configuration and main design figures are shown in Figure 3.
All operation data and presented analysis results are based on continuous sampling and process control reported from the different plants.

Results and Discussion
The WWTP plant built outside Colombo, Sri Lanka was initially (years 2011-2012) operated at load conditions, substantially lower than the design levels. Actions were taken to safeguard a feasible operation mode. The results were quite acceptable, and presented in a scientific paper, see [4], the results are summarized in the paper, quote: Comments to the actual load figures: As found in the table, the actual loads are typically less than 50% of design loads, except for BOD 7 being typically around 50% of design load.
Thus, in 2016 a test operation started at the plant with an intermittent aeration of the activated sludge. The typical scheme was as follows.
The operation cycle, covering both the aeration, non-aeration and withdrawal of excess sludge during the test period was 3 hours, with the following actions. • The aeration time was 120 minutes (2 hours). The organic variable, expressed as BOD 7 is understood as Biochemical Oxygen Demand, for seven days. This is an alteration within the Nordic and Baltic countries from the conventional BOD 5 , used in most countries. The basis is the 5-day working week in these countries.
• During the aeration time, waste activated sludge was removed from the biological step and chemical sludge from the chemical precipitation. This was necessary, as all internal sludge pumping was based on airlift pumping. • The sludge scraper in the chemical stage operated for 60 minutes during each cycle.
• The sludge scraper in the biological clarifier was operated continuously.
The results during the test period were better than expected, and the following main conclusions were drawn from the operation: The discharge values for the ruling permit variables were kept with a good margin, BOD 7 -concentration was almost consistently around 10 mg/l, compared with the consent level of <15 mg/l. The discharge P-level, presented as a mean value, was found to be 0.35 mg/l, compared with the ruling consent level of <0.5 mg/l. Consequently, the annual energy savings for the plant was 18,000 kWh, or 1/3 rd of the previous energy needs for the biological treatment step (55,000 kWh/ year).
Based on these findings PIREVA decided to evaluate a permanent operation of the Rosvik WWTP using an intermittent operation. In a longer perspective it would also be interesting to evaluate the potentials of an enhanced biological phosphorus removal at the plant. Some of the actions were included for the ongoing operation, and the intermittent operation continued. In early 2020, the discharge levels from the Rosvik WWTP were indeed very satisfying, as shown in Table 2.
Comments to the latest results: The discharge levels are all found as being more than required. These results have further encouraged PIREVA to consider a "twin plant" called Norrfjaerden to be converted into the same intermittent mode. Accordingly, the very good P-removal levels indicate a possibility to develop a certain biological P-removal level, see for instance findings earlier during similar climatic and geographical conditions [6].

Conclusions
The very long tradition of the activated sludge treatment model within the water industry has demonstrated very versatile possibilities to adopt the operation mode for different enhancements. By looking into other treatment models within the activated sludge family it is possible to find alternatives for the operation. Some important findings related to the intermittent aeration mode may be summarized as follows: • An energy savings for the operation, as described above; • Sometimes, also improved sludge settling characteristics thus providing improved discharge figures; There are however some needed conditions to accomplish these improvements: • Reliable on-line probes for both oxygen control, SS-concentration control and optionally also for nitrogen control; • A flexible automation system that allows the needed process modifications to take place; • And finally, very important dedicated and competent plant operators, with the needed curiosity for operation improvements.