Final Aeration of Excess Sludge with Excess Loading Process
Activated sludge processes are the most commonly used biological process for wastewater treatment. Although the process is widely successful, scope for improvement remains in addressing some issues - one of which is the high aeration energy requirement of the process. Whilst there are some studies on the reduction of aeration energy through lowering the sludge age, using fine-pore diffused air systems, and aeration process controls, energy consumption may be improved significantly by making use of microbial metabolisms.
For example, there are microorganisms that remove organic matters in wastewater and convert them to temporal carbon storage.They later oxidize temporal carbon storage for growth.Their metabolic formation of temporal carbon storage is useful to us as the conversion of organic matter in wastewater to temporal carbon storage requires less oxygen in comparison to conventional aerobic treatment. Furthermore, temporally stored carbon may be used as the source of biomass energy. As temporal carbon storage materials in activated sludge, polyhydroxyalkanotes (PHAs) are the most common form. In FAREWEL process also, PHAs are expected to be the most typical temporal carbon storage material.
It is feasable to use FAREWEL process, as most PHAs accumulated in FAREWEL process can be easily degraded and will be recovered as methane gas.
Table of Contents
Principles
In activated sludge there are microorganisms that accumulate temporal carbon storage materials. They may give us an opportunity to reduce energy consumption for aeration and energy recovery in sludge treatment. That is, the conversion of BOD to temporal storage materials require less energy, and thus BOD will be removed with less oxygen consumption. Furthermore, temporarily stored carbon will be converted to biogas in sludge treatment process.
In order to make use of temporal carbon storage phenomena we need to improve energy efficiency of activated sludge process. This treatment system (Figure 1) involves two stages-first is the conventional activated sludge process and second is the FAREWEL reaction where the excess sludge discharged from the first stage is mixed with wastewater (final feed)under overloaded condition.
Note:Microorganisms in excess sludge are usually “Hungry” after a long aeration. Why not feed it again to accumulate temporal carbon storage before you send them to sludge treatment?
Fig 2. The Storage of Organic Matter during the reduction of oxygen consumption and increasing recovery of biomass
Fig 2 shows the situation of active storage happens during the bacteria’s activity. Just after the treatment, the organic matter is removed and most of them are stored inthe microbial cell. In Fig 2, time A shows the optimum time is at the end of storage. In that case, the stored substances will be kept as the storage. However, normally we treat stored substances until the time B - then part of the stored substances will have been treated completely.
In the proposed FAREWEL (Final Aeration of Excess Sludge With Excess Loading) process, excess sludge has been kept as the overloaded status. In other words, if the stored substances are treated until A (in Fig 2), aeration could be controlled with the OA/OB level, and in the case of DO control, respiration amount could reduce to OA’/OB’ level. And for biomass recovery, the increase is about XY.
Application
Fig 3.The Components of New Process with FAREWEL Reaction
Fig 3 shows the change of in a wastewater treatment system before and after applying the FAREWEL process. Besides the normal aeration tank and secondary settling tank, a new aeration tank and secondary settling tank (for FAREWEL reaction) are added. In Fig 4. the design of an existing sludge treatment process compared to a FAREWEL process has been shown. In this FAREWEL process, the ratio of influent affects the effect of oxygen consumption and biomass recovery. Also the FAREWEL process is so dependent on the excess sludge which is produced from the normal processes. So, the ratio of QAS and QF is the most important parameter in the process design.
Simulation of Reduction of Oxygen Consumption and Confirmation of the Increase of Biomass Recovery
In order to find the amount of stored substances in bacteria cell, Acetate concentration ,PHA and oxygen uptake rate (OUR) are measured in the experiment.
YF is about 0.64~0.70,and the acetate uptake rate is about 4.5~5.1kgCOD/kgCODvss・ｄ
Benefits
With the modification of normal aerobic process by the FAREWEL process, aeration is reduced by about 10% , and biomass recovery is increased by about 30% . However, it is apparent that BOD removal could be a little bit affected. So after the treatment of a FAREWEL process, post-treatment is necessary to meet the effluent regulation - Wet-land system is a good option and is considered one of the cost-efficient post treatment systems of the FAREWEL process.
Also, it is necessary to discuss the influence of sludge treatment systems after the FAREWEL process. The evaluation on the degradation of excess sludge accumulating PHA under anaerobic digestion through excess methane production compared to excess sludge without PHA could be important. Excess sludge accumulating PHB caused more methane gas generation than excess sludge without PHB
Fig 4.The modification of Existed Process to FAREWEL process
References
1) Mamoru Oshiki,Hiroyasu Satoh,Takashi Mino.(2009). Aeration-Saving and Biomass Recovery Treatment through Temporal Carbon Storage and Adsorption Process by Activated Sluge. Journal of Water and Waste, 51(9):737-745(In Japanese)
2) S.M.Shamsul Huda, Hiroyasu Satoh, Takashi Mino(2010). Degradation of excess sludge accumulating polyhydroxybutyrate produced from energy saving activated sludge process under anaerobic digestion. International Symposium on Southeast Asian Water Environment.8:335-343
