start-up

Introduction

Concerning municipal wastewater treatment plants a start-up is usually a single event occurring only once in a life-time of the treatment works. Small seasonal treatment plants in the mountains have to restart their operation once or even twice a year and therefore the start-up phase represents a reasonable portion of the operation period.

In this context there is a significant difference between activated sludge and biofilm systems: The suspended solids of activated sludge systems are mobile and hence it is possible to inoculate such systems by adding an active biomass from a municipal wwtp.

Biofilm system

Since biofilms are built up by attached growth of bacteria, an inoculation of biomass is not practicable. During the start-up period a sufficient biofilm mass has to be developed – seeded and fed by the wastewater. Now the question rises whether the treatment capacity grows as fast as the loading. In order to investigate this issue the start-up of the trickling filter system at the Gera Refuge is analysed:

The Gera Refuge is extremely located (2324 m altitude) and difficult to approach (2 h walking distance). Therefore a rather slow increase of the loading at the beginning of the summer season is expected (Fig.1). In Fig.2 the profiles of organic and nitrogen compounds in the influent and the effluent flow are shown. Because of the relatively high base flow of about 2 m3/d even without any guests the concentrations increase significantly with increasing loads

Fig. 1: Profiles of temperature, hydraulic and organic loading of the trickling filter system Gera Refuge

Fig. 2: Influent and effluent concentrations of the WWTP Gera Refuge during the start-up period

Concerning organic compounds (COD and BOD) the treatment efficiency of the WWTP Gera Refuge achieves values of more than 90 % within 11 days. Also under increased loading during the following weeks this efficiency is obtained. Nitrogen elimination efficiency shows a delayed development. Measurement results from the 11. day of operation indicate an efficiency of 33 %, which is caused by nitrogen fixation into the biomass and not by nitrification. But after another week and an increase in water temperature from 6.4 to 10.0 °C nitrogen elimination improves to more than 60 %. Obviously during this period a sufficient population of nitrifying organisms has developed and a stable nitrogen elimination is achieved.

Fig. 3: Development of the treatment efficiency of the WWTP Gera Refuge during the start-up period

Activated sludge

The construction of the WWTP Stuttgart Refuge has been completed in the middle of the season 1999. In is an upgraded 3-chamber septic tank with a cyclic activated sludge system installed in the largest chamber (ESB continuous flow single tank technology). The two small chambers still serve as septic tanks. The design loading of the WWTP is 150 PE and already at the 3. Day of the start-up period a load of 112 PE was measured (Fig.4). The initial water temperature was 9 °C.

Fig. 4: Profiles of temperature, hydraulic and organic loading of the cyclic activated sludge system Stuttgart Refuge

The activated sludge system was inoculated by 200 l of pre-thickened activated sludge from a nitrifying municipal WWTP. After the tanks of the WWTP had been filled at the 4. day of the start-up period a suspended solids concentration of 0.34 g SS/l was measured (i.e. a sludge mass of 7 kg at a tank volume of 20 m3). After another 4 days the sludge concentration increased to almost 0.73 g/l. The mass balance of the conversion of 10.4 kg BOD influent load minus an effluent load of about 1.0 kg BOD to an activated sludge mass of 7.8 kg SS results in a biological yield of 83 %, 0.83 g SS/g BOD respectively.

Fig. 5: Influent and effluent concentrations of the WWTP Stuttgart Refuge during the start-up period

The presented concentration profiles in fig.5 show that inoculation enables a quick start-up also during a period of high loading. On the first day of a wastewater emission (4. day of the start-up period) the treatment efficiency achieves values of 92 % concerning BOD and 65 % concerning nitrogen (fig.6). Then the N-elimination decreases to 41 % after 4 days and 32 % after 12 days despite increasing sludge mass in the system. Obviously the air compressor did not work properly (had to be exchanged the next season) and could not supply enough air for a sufficient nitrification.

Fig. 6: Development of the treatment efficiency of the WWTP Stuttgart Refuge during the start-up period

Conclusions

Presented investigations show that activated sludge systems in the mountains can be started up immediately by sludge inoculation. The treatment capacity of Biofilm systems especially concerning nitrogen elimination develops slower. At many sites the loading at the beginning of the season will increase also moderately as shown in the case study. Generally the start-up of a newly installed biofilm system takes more time than the re-start in the following seasons because attached growth on fresh and smooth surfaces is retarded.

28. Mar 2002