Anaerobic pre-treatment of strong sewage : a proper solution for Jordan

The main objective of this research was to assess the feasibility of applying low cost anaerobic technology for the treatment of relatively high strength sewage of Jordan using two-stage and one-stage UASB reactors operated at ambient temperatures. The wastewater produced in Jordan is characterised by a high concentration of COD tot with averages higher than 1200 mg/l and with a large fraction in the suspended form (65-70%). The average wastewater temperature fluctuates between 18 and 25 oC for winter and summer respectively. The sludge bed in the UASB reactors was first simulated using CSTR systems. The objective was to study the digestion process as a function of temperature and SRT. Of particular interest was the assessment of the sludge potential to form a scum layer in relation to the degree of sludge digestion. The results revealed that methanogenesis starts only at an SRT between 30-50 days for reactors operated at 15 oC, while it starts at an SRT between 5-15 days for reactors operated at 25 oC. Both SRT and temperature affect the extent of scum formation. The degree of digestion has a clear effect on the concentration of lipids. Latter compounds tend to adsorb on sludge particles and have a strong tendency for floatation. However, it was found that sludge with a high scum forming potential only will produce scum in the presence of gas production. Based on these results scum formation in UASB systems could be prevented either by attempting to achieve a 'complete' conversion of lipids (one stage conventional UASB reactor with long SRT) or by preventing the evolution of gas production. The later could be achieved by designing a two stage UASB reactor, where the first stage mainly aims at the entrapment and partial hydrolysis of solids, while the second stage could act as a methanogenic reactor for the final conversion of the hydrolysed materials from the first stage. A 96-m 3two-stage UASB reactor was built at the location of Khirbit As-Samra treatment plant, which treats wastewater produced by 2.2 million inhabitants -almost half of the population of Jordan-. Operating the reactor for a year at 8+6 hrs HRTs for the first and the second stages respectivley resulted in average COD tot and COD ss removal efficiencies of 51% and 60% respectively for the first stage with no significant effect of temperature. The second stage had a poor performance and most of the treatment was attributed to the first stage. Biogas was produced in the first stage and resulted in heavy scum formation and sludge washout from the first to the second reactor, which affected the performance of the latter. Moreover, sludge produced in the first stage needs further stabilisation, particularly during wintertime. The performance of the first stage could be improved by enhancing solids removal using an AF reactor instead of an UASB reactor. An AF reactor was operated at an HRT of 4.6 hrs at 25 oC. The media in the filter are reticulated polyurethane foam sheets, which were vertically oriented in the reactor. Sludge was discharged regularly from the reactor. The results showed an average COD ss removal efficiency of 71%. The Operating the first stage reactor (60 m 3) as a conventional UASB reactor at an HRT of 24 hr showed an average removal efficiency of 62% for COD tot during summer. The removal efficiency dropped to 51% during wintertime. However, the effluent suspended solids were stabilised with a VSS/TSS ratio around 0.50 all over the year. Moreover, the sludge developing in the one stage reactor is well stabilised and exerts an excellent settlability. Regular sludge discharge from the one stage UASB reactor had no significant effect on the performance in terms of COD tot removal efficiency; however, sludge discharge most likely resulted in a more stable performance of the system, as wash out of scum layer sludge would remain low. The removal of the stabilised solids from the effluent of the UASB reactor will provide an average total COD removal efficiency between 87-93%.