The aim of this study was to determine elementary processes related to the organic and nitrogen compounds removal in hybrid reactors with intermittent aeration, to assess removal efficiency under various organic and hydraulic loadings and organic and nitrogen compounds’ utilization rates. The utilitarian aim was to determine technological conditions which could make the process shorter and more economically efficient. The attempts to modeling using various technical parameters (together or separately) were conducted.

3.1. Laboratory model and methods

Carriers used in the research were corrugated cylindrical rings made of PP diameter and length of 13 mm, 0.98 g/m3 density and 0.86 porosity (figure 5).

The research studies were conducted in four stages of 10 months duration. Each stage was consisted of three or four series. In each stage three reactors worked simultaneously as continuous flow system in stage I and III and as a sequencing batch reactor in stage II. The volume of reactors was equal to 75 dm3 and volume of settler for the continuous flow was equal to 20 dm3.

The studies were focused on an intermittent aeration. The most attention was put on the last stage — with increased wastewater pH value using lime (Ca(OH)2). The aim of higher pH maintaining was to inhibit the second phase of nitrification by ammonia. The wastewater originated from one family household. The retention time before the sewage discharging into the reactors was relatively high — about 6 days (septic tank and retention tank). The activated sludge originated from Poznan Central WWTP and was inoculated to each reactor at the same amount in each stage beginning. Mixing and aeration of the reactors was made using large-bubble diffusers. The air was supplied by compressor of 0.1-2.0 m3/h capacity. The sludge recirculation was made using an air-lift cooperating with a membrane pump. Characteristic research parameters are shown in table 2.

The characteristic feature of the used sewage was a low C/N ratio caused by pretreatment in a septic tank. The pollutants in sewage and suspended biomass concentrations were measured according to the standard methods. The attached biomass concentration was identified via the Kjeldahl nitrogen measurement: 1 g Ntkn corresponds to 0.11 gdm [50]; pH and oxygen were measured using calibrated electrodes.

The detailed description of research results of all experimental stages is included in the Makowska’s monography [11]. In this chapter only the most important processes and parameters related to the carbon and nitrogen removal efficiency are presented. Results related to the parameters like: biomass loading, pollutants’ removal efficiency and substrates utilization rates were analyzed statistically.