Characterisation and fluidisation of synthetic pit latrine sludge

Half of the world's urban population will live in informal settlements or ‘slums’ by 2030. Affordable urban sanitation presents a unique set of challenges as the lack of space and resources to construct new latrines makes the de-sludging of existing pits necessary, and this is something that is currently done manually, with significant associated health risks. Various mechanised technologies have therefore been developed to facilitate pit emptying, with the majority using a vacuum system to remove material from the top of the pit. However, this results in the gradual accumulation of unpumpable sludge at the bottom of the pit, which eventually fills the latrine and forces it to be abandoned. This study has developed a method for fluidising unpumpable pit latrine sludge, based on laboratory experiments using a harmless synthetic sludge. The implications for sludge treatment and disposal are discussed, and the classification of sludges according to the equipment required to remove them from the latrine is proposed. Finally, further work is suggested, including the ongoing development of a device to physically characterise latrine sludge in-situ within the pit.

Ozonation of endogenous residue and active biomass from a synthetic activated sludge

Coupling the activated sludge and the ozonation processes is an efficient, although expensive, solution for sludge reduction. A better knowledge of the mechanisms involved in the degradation of various sludge fractions by ozone is needed to optimize the coupled process. The objectives of this study were to determine the biodegradability of ozone-solubilized endogenous residue, the action of ozone on the active biomass and the solubilization yield of these two main sludge fractions. Batch tests were conducted with slug input of ozone stock solution into fresh or aerobically digested synthetic sludge. Biodegradability of the solubilized endogenous residue was increased by ozonation by up to 0.27 g BOD5/g CODi. Ozone caused biomass lysis, as opposed to an increase in maintenance needs, as shown by a correlation between the decrease in microbial activity and viability. Lysis caused by ozonation was associated with a solubilization of 20% of the lyzed cell COD mass. Solubilization yields were of 9.6 and of 1.9 to 3.6 g COD/g O3 for fresh and aerobically digested sludge, respectively. Design of sludge ozonation processes should account for the variability between the solubilization yield and biodegradability of the various sludge fractions.

FLOCCULATION DYNAMICS OF SYNTHETIC AND ACTIVATED SLUDGE IN WASTEWATER TREATMENT

This study has investigated in a comparative fashion the effect of cation, polysaccharide, and Polyelectrolyte on the flocculation dynamics and final properties of both synthetic and activated sludge. The results from experiments indicate that cation, polysaccharide, and Polyelectrolyte could influence sludge floc characteristics as they relate to sludge conditioning. The relationship between polysaccharide and cation concentration was examined at laboratory scale during flocculation with both synthetic and activated sludge; an increase in feed cation concentration led to a decrease in final polysaccharide concentration of the supernatant. The effects of a polyelectrolyte conditioner on sludge conditioning were determined. The results indicated that the cationic Polyelectrolyte had the critical effect on sludge dewatering, and made the improvement of the final properties of sludge. A qualitative link exists between polyelectrolyte conditioner dosage and sludge conditioning for both types of sludge. The two types of sludge also have very similar sludge conditioning. Thus synthetic and activated sludge behave very similarly in terms of their characteristics and sludge conditioning, and synthetic sludge can be used as a surrogate in activated sludge studies.

The preparation of synthetic sludge for lab testing

For carrying out sludge lab-scale tests, and interlaboratory trials for standards validation, the availability of samples with “certified and constant” characteristics is required. When “dried sludge” samples cannot be used, problems arise because (i) most fresh sludge characteristics change with storage time, also because some preservation practices are not applicable or their effects unknown, (ii) some fresh sludge characteristics are strongly affected by handling, and (iii) fresh sludge requires particular precautions and authorization for transportation. This means that using “fresh sludge” samples cannot guarantee reliability and reproducibility of results, so a valid alternative consists in testing “synthetic suspensions” samples to be on-site prepared. This should also give the possibility to compare results obtained in different places and times. Within this framework, Task Group 3 (TG3) of Working Group 1 (WG1) of Technical Committee 308 (TC308) of European Standardization Committee (CEN) undertook the preparation of a Technical Report dealing with the preparation of synthetic suspensions, both inorganic and organic, in repeatable conditions, able to describe the behaviour of a real sludge. This paper discusses a proposal for ingredients and modalities to prepare such synthetic suspensions that will be systematically tested in already planned lab experiments to define an optimal recipe.

The importance of structural and gel fractions in determining shear sensitivity of sludge

Shear sensitivity measurements with variation of ingredients in a synthetic sludge can be related to biosolids' structural properties and particulate amount. The shear sensitivity is primarily controlled by the gel fraction, with alginate and the associated cations increasing the gel-like behavior. However the effect of particulate fraction on shear sensitivity should also be taken into account. Shear sensitivity is thus a complex parameter including two types of response: the resistance to dispersed mass concentration into centrate, and the maintenance of desired strength while the solids concentration increases in the shear field.

Inhibition Effects of Inorganic and Polymer Coagulants on Acidogenic Fermentation

Laboratory-scale batch experiments were conducted to investigate the possible inhibition of acidogenic volatile fatty acids (VFAs) fermentation by an inorganic and a polymer coagulant. It was found that the conversion from organic matter to VFAs decreased more than 10% when the dosages of coagulant exceeded 46 (mgAl/L) and 120 (mgFe/L). Inhibition by Al was greater than that Fe. When the dosage of polymer was over 102 (mg/L), VFAs conversion fell to below 2%. The protein content was more affected than the carbohydrate content. Ultrasonic treatment increased the VFAs conversion for synthetic sludge at both treated with either inorganic coagulant or polymer