ACTUACIONES PREVENTIVAS DE SEGURIDAD SOCIAL

The ammonia removal efficiency and removal rates of the bench and test pilot-scale systems before the addition of PMR wastewater showed no significant statistical difference despite the two units being operated with different domestic feed. However, when assessing the removal efficiency of both systems operating at steady state, there was a statistically significant difference between the systems. The pilot-scale test unit maintained an average removal efficiency of 15.45 % while being operated at 2 % PMR wastewater (v/v), while the removal efficiency of the bench-scale unit never exceeded 4.13 %. 0 200 400 600 800 1000 1200 0 50 100 150 200 250 300 C h lo ri d e c o n c e n tr a ti o n ( m g .L -1) Time (days)

CHAPTER 5

The bench scale activated sludge process was scaled up according to the volumetric ratios of the unit, hence the geometric ratios of the aeration tanks and clarifiers could not be maintained. The bench scale system exhibited good sludge recycling and no rising sludge in the clarifiers, which ensured that the AS had a consistent supply of oxygen and nutrients and resulted in suitable biomass turn over. In contrast, the design of the pilot-scale clarifiers compared to the bench scale clarifiers (Figure 5.8) resulted in a loss of biomass as the AS became trapped in the clarifier without an oxygen supply and fresh feed for more than a day at a time. Clarifier design is concerned with efficient removal of SS from the effluent. The hydraulic characteristics of full scale plants cannot be simulated at bench scale owing to typical hydraulic overflow rates and solids loading rates, but even so some aspects of design are worth considering. The base of the clarifier should be conical to a flat bottom to minimise short-circuiting of RAS as it is drawn to the aeration basin. Protrusion of the RAS intake up into the clarifier, or the use of a conical bottom, promote sludge accumulation at the very bottom of the clarifier and leads to rising sludge (Jenkins et al., 1983), as observed in the pilot plant.

Figure 5.8 Schematic comparison of the bench (a) and pilot scale (b) clarifiers. Not drawn to scale.

Despite the effects of localised deoxygenation and nutrient starvation on the biomass, the pilot scale unit was able to continue nitrification in the presence of the PMR wastewater. This suggests that nitrification in the bench scale unit may have been affected more by the operational pH than the toxicity of the PMR wastewater. If the feed pH could be increased further, acclimatised AS may nitrify more of the ammonia present in the wastewater. This may be augmented with carbon and phosphorus addition to sustain biomass growth and ensure that low nutrient concentrations are not responsible for limiting metabolic function. The observation also suggests that the clarifier design problems which caused short-circuiting may also have allowed anaerobic zones to develop, thus facilitating nitrification-denitrification to begin inside the clarifier.

CHAPTER 5

In contrast to the ammonia removal, COD removal in the pilot-scale unit was observed to be lower than that of the bench-scale units, despite the lower PMR wastewater concentration. Since autotrophic organisms are more sensitive to environmental changes than heterotrophs, it would be anticipated that the improved ammonia removal of the pilot-scale unit would correlate with a similar improvement in COD removal when compared to the bench-scale units. The fact that this was not observed further highlights the potential negative effects of the design of the pilot-scale system.

5.4 Conclusions

One of the most significant factors influencing the functioning of the pilot-scale system was the inappropriate clarifier shape. Biomass was lost in both test and control units and this exacerbated the effects of the PMR wastewater on the test unit. Despite this, the test unit was able to maintain better nutrient removal than the bench-scale AS units as a result of the increased operational pH at the lower PMR wastewater concentration. The use of aeration to provide mixing in the aeration tanks and the lack of an anoxic zone precluded full nitrification-denitrification. Modification of the pilot plant to insert either impellers (and decrease the aeration rate) or baffles to pre-contact the RAS and the feed in an unaerated zone could considerably improve nitrogen cycling. This indicates that improved AS functioning may be achieved by optimising the process design.

CHAPTER

CHAPTER

CHAPTER

CHAPTER 6666

M

MM

MATURATION OF WASTEWAATURATION OF WASTEWAATURATION OF WASTEWAATURATION OF WASTEWATER PRIOR TO BENCH STER PRIOR TO BENCH STER PRIOR TO BENCH STER PRIOR TO BENCH SCALE CALE CALE CALE AS,AS,AS,AS, ALGAL ALGAL ALGAL ALGAL

AND COMBINED TREATME

AND COMBINED TREATME

AND COMBINED TREATME

AND COMBINED TREATMENT PROCESSESNT PROCESSESNT PROCESSESNT PROCESSES

6.1 Introduction

Within the context of domestic wastewater treatment, algae have been utilised to treat both raw and settled wastewater in primary or secondary facultative ponds or as high rate-algal ponds (HRAPs) (Mara, 2003). The main function of the algae in facultative ponds is removal of BOD, nitrogen and phosphorus. The advantages of using algae in an effluent polishing step are the low cost of operation, discharge of oxygenated effluent into the receiving water and nutrient removal (Aslan and Kapdan, 2006). Furthermore, algal processes may be applied to wastewaters containing low concentrations of carbon as algae utilise carbon dioxide, carbonates and bicarbonates as dissolved inorganic carbon sources (Antunes et al., 2003).

Algal technology has been applied to the direct treatment of agro-industrial wastewaters (Ogbanna

et al., 2000; An et al., 2003; Olguín et al., 2003; Mulbry et al., 2008), paper and pulp wastewaters

(Dilek et al., 1999; Tarlan et al., 2002) and indirectly by blending effluents for the treatment of acid mine drainage (AMD) (Whittington-Jones et al., 2006). The use of algae for removal of industrial contaminants such as heavy metals, hydrocarbons, surfactants and biocides has been widely investigated; however, while the studies demonstrate the capacity of algal technologies to treat these compounds, actual potential for application has yet to be realised (de-Bashan and Bashan, 2010).

6.1.1 Rationale

During operation of the pilot plant it was noted that algae grew on the clarifier walls. Further, sludge with some apparent anaerobic activity accumulated at the bottom of the pilot plant test-unit feed tank, and the pH of the bulk liquid feed increased slightly during intervals between each fresh feed supply. A simple flask test was carried out to determine whether sludge extracted from the feed tank could increase the pH of fresh feed. Three 1 L flasks of mixed feed were inoculated with 100 ml of sludge from the pilot-scale test unit feed tank and topped up with the domestic and PMR wastewater blend. An identical control flask was filled with the domestic/PMR wastewater blend, without sludge inoculation. The flasks were covered to minimise evaporation and the pH was monitored twice a week for three weeks with a pH electrode (Hanna Instruments, USA). Figure 6.1