Thickening Ponds

Faecal sludge is not a uniform product and, therefore, its treatment must be specific to the characteristics of the specific sludge. Generally, such sludge does not have a higher solids content than 2 to 10 % (SASSE & BORDA 1998). Sludge, which is still rich in organics and has not undergone significant degradation, is difficult to dewater. Conversely, sludge that has undergone significant anaerobic degradation, is more easily dewatered.

In order to be properly dried, fresh sludge rich in organic matter ( high-strength sludge (HEINSS et al. 1998) e.g., latrine or public toilet sludge) must first be stabilized. Allowing the sludge to degrade anaerobically in sedimentation / thickening ponds which provide a long hydraulic retention time of several days or a few weeks can do this (STRAUSS & MONTANGERO 2002) . The same type of pond can be used to thicken sludge which is already partially stabilized ( low-strength sludge (HEINSS et al. 1998) e.g., originating from Septic Tanks , anaerobic digesters, biogas settlers, trickling filters or activated sludge processes , etc.), although it undergoes less degradation and requires more time to settle. The degradation process may actually hinder the settling of sludge because the gases produced bubble up and re-suspend the solids.

The ponds may be differentiated in thickening ponds for the treatment of fresh sludge (high-strength sludge, settling tanks) and thickening ponds for the treatment of low-strength sludge (sedimentation ponds).

As the sludge settles and digests, the supernatant must be decanted and treated separately. Appropriate post-treatment may be waste stabilisation ponds (optionally together with other wastewater) or soil infiltration (e.g. in soak pits, or leach fields) depending on the degree of pollution. The thickened sludge occupies much less volume and can be more easily transported. However, it can still contain pathogens and needs to be further treated. The thickened sludge can then be dried or further composted before reuse, disposal or incineration. Typically, sludge from thickening ponds is transformed into biosolids on drying beds or co-composted together with organic solid waste (co-composting small or large-scale). In some cases, it may also be spread out in thin layers on the ground .

Two tanks operating in parallel are required; one can be operated, while the other is emptied. Both sedimentation ponds and settling tanks are designed based on the storage volume required for a desired depth and quantity of accumulating solids (STRAUSS & MONTANGERO 2002) in a given time (depending on the frequency of desludging). The specific volume occupied by separated solids may vary from 0.02 (thin sludge) up to 0.15 (high-strength sludge) m3/m3 of raw sludge depending on the type and composition and on the time allowed for solids consolidation and thickening (HEINSS et al. 1998; STRAUSS & MONTANGERO 2002). To achieve maximum efficiency, loading and resting periods should not exceed 4 to 5 weeks, although much longer cycles are common. When a 4-week loading and 4-week resting cycle is used, total solids (TS) can be increased to 14% (depending on the initial concentration).

Sedimentation Ponds

Sedimentation ponds are designed similar to anaerobic ponds in waste stabilisation ponds with a sufficient volume for sludge accumulation over a long period (KLINGEL et al. 2002). The anaerobic digestion process, which stabilises the fresh sludge, causes gas production (MONTANGERO & STRAUSS 2004). When the gas bubbles rise, this can further limit the sludge settling as solids are also pushed to the surface, but the produced biogas can be collected and used for heating or cooking (i.e. by covering the pond with a gastight canvas). Sedimentation ponds can be operated in batch and continuous mode and sludge is removed only once every 8 to 12 month (STRAUSS & MONTANGENRO 2002). Therefore they require generally more space than simple settling ponds. The organic load of sedimentation or anaerobic ponds vary from 250 to 350 gBOD/m3 per day and the volume of accumulated sludge per incoming solids may vary between 0.8 to 2 L/Kg TS (KLINGEL et al. 2002).

Settling Tanks

Settling tanks need not only sufficient volume for sludge accumulation, but also sufficient depth of the liquid column (> 1.5 m) in order to allow a fast and good settling (KLINGEL et al. 2002). The loading/resting cycle in settling tanks is much shorter than in sedimentation ponds and sludge is removed about every 2 to 4 months (STRAUSS & MONTANGERO 2002). The tanks should be equipped with baffles to maintain hydraulic conditions favourable to good settling and to retain floating scum (KLINGEL et al. 2002).

For design, accumulating sludge volumes per incoming solids load of 5 to 9 L/kg TS can be considered. However, the consistency and concentration of faecal sludge is highly variable (depending on storage duration, tank emptying technology and pattern, performance of treatment of collection unit, added matters, temperature, intrusion of ground- or surface waters, etc. (MONTANGERO & STRAUSS 2004) and treatment systems should be designed on a case-to-case basis (EAWAG/SANDEC 2008). 

Whether a sedimentation pond or a settling tank is used depends on the consistency of the sludge (e.g. fresh vs. old sludge), the space available and the means of sludge removal.

When only little space is available, fresh toilet sludge may be diluted with more stabilised sludge in order to be treatable in settling tanks (KLINGEL et al. 2002). It is also important to consider that the quantities of solids, which are produced and have to be removed in settling ponds, are much smaller than the mass of solid to be removed and handled from sedimentation ponds (EAWAG/SANDEC 2008). But frequent sludge removal can also become expensive for operators.

The removal rates, which may be expected in sedimentation ponds and settling tanks are (STRAUSS & MONTANGERO 2002):

Both the incoming and thickened sludge are pathogenic; therefore, workers should be equipped with proper protection (boots, gloves, and clothing). The supernatant liquid can be polluted with microbes as well and needs to be correctly treated or infiltrated. The thickened sludge is not sanitized and requires further treatment before disposal. Ideally, this technology should be coupled with a subsequent treatment technology if a hygienic product is required.

The ponds may cause a nuisance for nearby residents due to bad odours and the presence of flies. Thus, they should be located sufficiently away from residential areas.

Pond systems are relatively low cost, as they do not require much maintenance and no energy. However, large surface areas must be available and they need to be designed and operated by skilled staff. Depending on the frequency of desludging, emptying may become expensive or labour intensive.

Maintenance is an important aspect of well-functioning ponds, but it is not intensive. The discharging area must be maintained and kept clean to reduce the potential of disease transmission and nuisance (flies and odours). Solid waste that is discharged along with the sludge must be removed from the screen at the inlet of the ponds.

The thickened sludge must be mechanically removed (with a front end loader or other specialized equipment) after it has sufficiently thickened every 8 to 12 or 2 to 4 months for sedimentation ponds or settling tanks respectively (STRAUSS & MONTANGERO 2002) .

Trained staff for operation and maintenance is required to ensure proper functioning (TILLEY et al. 2008).