Black & Veatch's Frank Rogalla, with co-author Grzegorz Bujocze, looks at handling solids for low nutirent residualsThe main goals of any sludge treatment scheme are stabilisation and concentration of the sludge solids prior to final disposal. The reduction of the sludge liquor returns to the mainstream sewage treatment process is a side goal. In biological nutrient removal (BNR), relatively low levels of nutrients in the sludge liquors have to be achieved. The management of the surplus activated sludge prior to stabilisation aims at minimising the phosphorus release back to the liquid phase. In the subsequent stages of sludge treatment the effort is to bind the nutrients in the solids to eliminate the need for additional side stream treatment of the sludge liquors.
The nitrogen and phosphorus loading of the untreated return liquors from sludge processing has a tremendous impact on the mainstream sewage treatment. This is why nutrient management strategy must be established, along with the design of solids treatment works to minimise the impact of the return liquors.
Surplus sludge (SAS) quantities depend on the sludge age in the system, the average temperature and strength of the incoming sewage. Nitrogen in the SAS is about 6-10% of the volatile suspended solids (VSS). Any biological solids reduction process applied to BNR sludge will lead to solubilisation of nitrogen and phosphorus from cell lysis. The extent of the release will largely depend on handling of the sludge prior to stabilisation, conditions during the stabilisation stage and sludge dewatering.Sludge Thickening
The most common thickening for raw primary sludge is by gravity, where the solids underflow levels are in the range of 5-10% dry solids (DS). In BNR works, thickening can be optimised to aid the mainstream process, especially when the domestic sewage is weak and has low COD/P ratio (i.e. <40:1). Pre-fermenting the sludge will produce short chain volatile fatty acids (VFA) that are then returned in the supernatant stream to the activated sludge process. This readily available organic carbon source supplements the carbon in the raw sewage for Bio-P removal and/or denitrification. For a single-stage fermenter or thickener the loading rate ranges from 25-40kg/m2d and the sludge age is between four and eight days, depending on temperature. Although it may offer the least costly approach, gravity thickening on SAS will certainly lead to release of phosphorus.
Long retention times, mostly under anoxic conditions, promote the release of the accumulated cell-bound phosphates back to the liquid, which is then returned to the mainstream sewage treatment process. Typical retention time of a few hours is long enough for polyphosphate hydrolysis but too short for subsequent precipitation. Dissolved air flotation (DAF) is a viable alternative to gravity thickening because it offers a shorter retention time (<30 min) under aerobic conditions, thus preventing the re-release of phosphorus to the liquid. The most important design parameter is the air-to-solids ratio that should be higher than 0.03kg/kg dry solids.
Typical solids capture rate is between 96% and 98% and DS content in the thickened slurry up to 3.5%. Gravity belt, rotary drum or centrifuge thickeners give superior thickening capability up to 6-7% dry solids, usually involving polymer addition at doses from 2-5kg/tonne DS. This ensures high solids separation up to 99% and low SS in the return liquor.
Of all available options, centrifuges offer the smallest footprint and highest thickened sludge output of 8% DS but are the most costly. Gravity co-thickening of raw primary sludge and SAS has also been in use. This method is certainly not advocated for BNR works because it will create conditions similar to those in the anaerobic zone of the bioreactor, favouring the phosphorus release. As a compromise, gravity thickening of primary sludge is often combined with DAF or mechanical thickening of SAS. Innovative Pre-Treatment There is a choice of innovative pre-treatment technologies aimed at enhanced solubilisation of the organic matter in sludge prior to stabilisation or pathogen control:
- systems based on sonication, thermal hydrolysis and mechanical disintegration for solids disintegration. In a BNR works, the nitrogen and phosphorus released during these treatments needs to be dealt with in the same way as return streams following dewatering,
- pasteurisation for pathogen destruction relies on intermittent aeration of the sludge to aid autothermal heat generation and maintain aerobic conditions.
The most common system is thermophilic aerobic digestion (TAD). Its popularity is mainly due to the disinfection effect achieved in the thermophilic temperature range and improved dewatering characteristics. Nitrification does not occur at these high temperatures. Aerobic digestion at mesophilic temperatures in a sequencing batch reactor leads to nitrification during aeration phase and denitrification during the fill phase.
In cases where there is insufficient alkalinity present in the system, only partial nitrification takes place leading to ammonia returns in the recycle stream. This poses additional loading on the mainstream process in the bioreactor. Alkalinity adjustment and extended anoxic periods are advocated to increase nitrate utilisation rates. The extent of phosphorus release in aerobic digestion is insignificant compared to that under anaerobic conditions and is mainly due to bacterial cell lysis. The released phosphorus is usually taken up by the next generation of bacterial population in a digester.ANAEROBIC DIGESTION
In this widely applied stabilisation method the sludge organic matter undergoes multi-stage mineralisation. Sludge characteristics and operational conditions influence the degree of solubilisation and chemical balance in a digester. About 50% of the TKN removed from the sewage treatment stream in the raw primary sludge and the SAS can be hydrolysed and returned to the mainstream. Organically bound nitrogen undergoes ammonification contributing to the supernatant ammonia concentrations in the range from 800-1,200mg/l as N. Up to 80% of cell-bound phosphorus in the SAS can be released to the liquid (200mg/l as P) due to the breakdown of the bacterial cells through acid fermentation. Under favourable conditions both ammonia and phosphates can precipitate to magnesium ammonium phosphate (MAP, MgNH4PO4¥6H2O1), or struvite.
Controlled precipitation of this common compound is the key practice preventing high nitrogen and phosphorus loadings in the return liquors. MAP formation depends on the rate of solids degradation and subsequent magnesium ion release and the rate of polyphosphate hydrolysis to orthophosphate with released magnesium. Usually both ammonium and phosphate ions are in over-supply compared to magnesium. The pH of 5.5 is the practical starting point for struvite formation. Under alkaline conditions prevailing in a well operating digester struvite solubility is greatly reduced (100mg/l at pH of 7.5).
Phosphate can undergo precipitation to other stable salts. Primary sludge presence enhances phosphorus retention through dilution of orthophosphate from hydrolysis of polyphosphate-rich SAS and by supplying calcium that becomes available for phosphate precipitation to calcium phosphate. Struvite precipitation in a digester is difficult to control. Nevertheless, as long as the mainstream process operation is optimised and a proper chemical balance exists this phenomenon will take place. The key element lays in preventing struvite deposits in the pipework and downstream dewatering equipment, as remediation is laborious and causes excessive system downtime.
Proper hydraulic design must focus on prevention of turbulence in the system. This is linked to a decrease in pressure at pipework bends and other elements causing localised pressure loss. Decreased pressure leads to the release of carbon dioxide from solution, which increases the pH thus creating suitable conditions for struvite precipitation. There are cases where the extent of spontaneous nucleation and growth of struvite crystals in a digester is insufficient to achieve low levels of ammonia and phosphate in the return liquors. One of the available options is controlled struvite crystallisation in a separate stream by adding magnesium chloride or magnesium sulphate. From the nutrient control perspective, the best practice is not to return any supernatant from the anaerobic digesters. It is easier to apply treatment to the whole digested sludge stream or to treat dewatering liquors.Post-Digestion Handling and Dewatering
Another option for phosphorus control is precipitation with calcium ion to hydroxyapatite (HAP or Ca5(PO4)3OH1). When used downstream of anaerobic digestion and prior to the dewatering stage, this operation also provides an effective tool against uncontrolled struvite precipitation in the system. Lime is the most common additive that not only binds phosphate, but also effectively aids the dewatering process.
It must be stressed that excessive liming of the sludge can lead to scaling of dewatering equipment requiring frequent plant shutdowns for cleaning. Aeration of the anaerobically digested sludge prior to lime addition reduces the demand for lime by stripping carbon dioxide off the liquid and raising the pH. Lime addition to the return stream is also practiced, with ferric or alum used as alternative chemicals.
Popular sludge dewatering alternatives include centrifugation or filtration using belt filter or membrane presses. Alongside thickening this operation is the next main contributor of the liquors recycled to the mainstream sewage treatment. Depending on the preceding treatment stage, the dewatering liquors can contain large quantities of ammonia or phosphates.Side-Stream Treatment of Liquor Returns
Nutrient levels in liquors from thickening and dewatering operations can be reduced with physical-chemical or biological treatment methods. Struvite precipitation is among the most advocated methods. Normally, sodium hydroxide (NaOH) is added to the precipitation tank to increase the pH to around 9.2 and. A narrow pH range favouring struvite formation must be maintained as other parallel chemical reactions leading to the formation of magnesium hydroxide, magnesium hydroxyphosphates or magnesium hydrogen phosphate have higher formation rates at pH levels close to that for struvite.
When precipitated from the digester supernatant or centrate in a separate treatment stage struvite can be granulated and sold as a fertiliser. The release of phosphorus, nitrogen and magnesium in soil environment progresses gradually over an extended period of time, unlike in the case of many conventional fertilisers. Removal of ammonia in recycle streams from sludge treatment can also be achieved via nitrification. The additional cost of side stream treatment is justified when the existing bioreactor capacity or available space for extension is limited. Excess biomass from the side stream process can be recycled to the nitrification mainstream and reduce the size of the zone. High-rate biological return liquor treatment are the Moving Bed Bioreactor at Shoreham and the Membrane Bioreactor at Glasgow.