Aecomix^TM for efficient energy recovery from wastewater and waste streams

In parts of the food and beverage industry many high rate reactors are installed, typically on effluents with high dissolved organic matter and low total suspended solids (TSS) and Fats, Oils and Greases (FOG). These high rate reactors, such as UASB (Up flow Anaerobic Sludge Bed) and EGSB (Enhanced Granular Sludge Bed) reactors depend on the granulation of the consortia of biomass in the reactor. In many occasions there are problems with maintaining granular sludge, due to the presence of solids, oils and greases or other inhibiting factors. (Dereli et al. 2012). Wastewater with high levels COD, TSS and FOG can be found in many parts of the food and beverage industry, such as ice cream, chocolate, candy, vegetable, dairy and cheese factories. Also Palm Oil Mill Effluent (POME) falls in this category.

Traditionally, the treatment process for such factories consist of a pre-treatment step, typically a physical-chemical treatment, to remove the free fats and solids, sometimes followed by high rate anaerobic treatment, then generally followed by an aerobic biological treatment plant to achieve the discharge requirements. The pre-treatment step is essential to produce an effluent which is suitable for the aerobic micro-organisms. Known issues with not properly pre-treated effluents are: foaming, sludge bulking, grease layers on aeration tanks, and poor settling characteristics (Jenkins Richard & Daigger 2004). The result of a process with chemical pre-treatment is a good quality effluent, but it also produces a chemically treated sludge from the pre-treatment and excess aerobic biological sludge.

In the past decades the sludges from pre-treatment systems have become an ever increasing problem to dispose at ever increasing disposal costs. Decades ago these organic sludges could still be spread over land or mixed with other products to produce fodder as food for animals (pigs). More recently, in various countries, many of these sludges have to be incinerated. Lately, there is a trend to use these sludges as co-substrate in anaerobic digestion plants, yet still at a cost to the factory producing it. Besides these sludges from the effluent treatment processes, food factories are also producing organic wastes, which can either be rejected batches, returned products, concentrates produced during CIP cleaning, spills, etc.

To deal with the sludges of pre-treatment systems (physical-chemical) anaerobic digestion systems are often installed. In these cases chemicals and equipment are needed to produce these sludges. An interesting alternative is to mix the wastewater and organic waste together and treat it in an anaerobic digester. Since the wastewater is relatively diluted and the anaerobic digestion requires a solids retention time (SRT) of at least 15–20 days (Al Seadi et al. 2008), such digestion plant would require a retention time equal to the SRT. However, a reactor with a retention time of over 10 days is generally not economical.

The Aecomix^TM process is the solution to treat both waste effluent and waste solids in a single process, with a high efficiency and a rather compact design. Key to the Aecomix^TM process and success is a solids-liquid separation process under anaerobic conditions to return the biomass to the reactor. In this manner the SRT can be extended largely over the Hydraulic retention time. Typically the solids-liquid separation is a Dissolved Biogas Flotation (DBF) system, in some cases other separators like cloth filters or membranes may be required or used. In this article we will focus on the process with a DBF system. The Aecomix^TM process is beneficial for those fluids for which the SRT needs to be substantially higher than the HRT to obtain a stable and good process.

The aim of this paper is to show the possibilities of the Aecomix^TM as pre-treatment of wastewater with high concentrations of COD, TSS and FOG (Standard Methods). A full-scale Aecomix^TM was tested on the effluent of a chocolate/candy factory. Also an assessment is made of the operating expenses (OPEX) compared to the above described traditional concept consisting of full chemical treatment followed by high rate anaerobic treatment (UASB) for water and a digestion plant for the sludge phase.

First some typical examples of the composition of organic waste and wastewater are presented which are very well suited for the Aecomix^TM process, followed by a case which will show the performance of the Aecomix^TM process.

Typical examples of organic wastes of different industries are shown in Table 1.

In certain countries pre-treatment (hygienisation) may be required for wastes which are mixed with animal products, to get the anaerobic process approved. Pre-treatment steps may include: shredding, cutting, pasteurizing, sterilisation, hydrolysing and solubilising. (Schnürer & Jarvis 2009).

Typical wastewater values for the above mentioned industries are outlined below (in practice this will strongly depend on type of products produced and production and cleaning methods):

The FOG and/or TSS figures of the wastewaters shown in Table 2 prove that these effluents are not (very) suitable to treat in UASB/EGSB type reactors, without pre-treatment. Such pre-treatment will have to include chemical treatment in the form of coagulation-flocculation. This means that provisions have to be made for chemical storage, chemical dosing and reaction tanks, followed by a solid/liquid separation process. This does not only require investment and operational costs, but also creates a concentrated flow which needs to be disposed of or treated as described earlier.

The method proposed herein intends to overcome these issues and create a rather simple, but robust approach to the treatment for this type of wastewater. On top of that it will show that the organic wastes can also be added to the process, which makes this solution practical and very attractive to these types of industries.

The organic wastes may be liquids, slurries and even (in case of chocolate factories) solid blocks. Generally these wastes need to be collected at the point of production and transferred to the site of the treatment plant. Here they are to be pre-treated to create a liquid mix which can be pumped to the reactor or pre-mixed with the wastewater. This pre-treatment is very much dependant on the type of product, the country (regulations) and the process requirements and is therefore kept outside the scope of this article.

The wastewater which is strongly organic loaded, may also contain larger debris, such as cloth, pieces of wood (ice-cream sticks), paper. As a first treatment step a screen will remove these debris. Typically such screen is installed in a channel. Wastewater is collected in a pump sump or (mixed) equalisation tank to enable the system to feed wastewater over 24 h to the Aecomix^TM reactor. The organic wastes are proportionally fed to the equalisation tank to create a consistent mixture.

In case an aerobic biological treatment plant is following the Aecomix^TM process, the excess sludge produced in the aerobic system can be fed into the Aecomix^TM system as well. The bio solids will be partially anaerobically degraded, resulting in less solids output and only one type of sludge to be disposed of.

The Aecomix^TM typically operates at mesophilic (approx. 37 °C) conditions (thermophilic at 45–60°C is also possible). In case the raw wastewater is much lower in temperature, it will be advantageous to use a heat recovery system. This is typically a heat exchanger in which fresh effluent is pre-heated by the treated effluent. In this case the wastewater is pumped from the equalisation tank to a heat exchanger, from where it will fed into the anaerobic reactor.

In Table 3 the composition of the substrate is given as it enters the anaerobic reactor, based on the wastewater volume, organic waste quantity and excess sludge quantity.

The anaerobic Aecomix^TM process comprises of one or more complete mix reactors. Typically there will be one reactor, or two reactors which can be in different configurations. The selection and configuration depend on the scale, requirements, type of substrates and economics. The standard tank is circular, fitted with a double foil membrane roof. In case of more than one reactor, the first reactor may be a tall reactor with a fixed roof and the other reactor will be fitted with the membrane roof for gas storage. In case of a membrane roof there will be a net above the liquid level, to support the membrane when there is no gas present, and to avoid that the membrane is submerged in the fluid. The net also functions as a first (limited) step in desulphurisation as Thiobacillus will grow on the net and capture the H₂S in the biogas (Lee et al. 2011). In some instances this will be sufficient, in other cases additional gas scrubbing facilities are added.

Micro-organisms in the reactor will convert the organic matter to volatile fatty acids and methanogenic bacteria will consume these acids to produce methane. The biogas which is generated in the fluid will move to the surface and is collected in the gas phase. This produces a first form of continuous mixing in the reactor. To enhance the mixing and avoid stratification, mixing devices are installed in the reactor, which typically only operate intermittently (Blaschek Ezeji & Scheffran 2010). The temperature in the reactor is controlled by steel tubing's alongside the wall, which act as a heat exchanger. The retention time may vary between 1–14 days. The organic loading will depend on the substrates and requirements and will typically be between 3–7 kg COD/m³/day (Schnürer & Jarvis 2009). The recycling of solids is required in order to achieve this organic loading and to attain a sufficiently long SRT to allow the micro-organisms to sustain and grow in the system.

The mixed liquor is discharged to a solids-liquid separator. Normally this is a DBF device, alternatively it may be a gravity thickener or a membrane system. Micro-organisms in the anaerobic system are sensitive to air and shear. It is therefore important that the solids-liquid separation will be a process with low shear (Schnürer & Jarvis 2009) and that there is (almost) no contact with air. The bio solids are returned to the reactor and some excess bio solids are disposed of.

The DBF device is the essential part in the process (Strasse & Stoffregen 1981). It meets all the requirements for optimum treatment of the bio solids (gentle treatment, no contact with air), it is a robust system and cannot get clogged. The DBF device operates much like a dissolved air flotation (DAF) process, with the exception that air is replaced with biogas and the biogas is dissolved in a different way from the standard solution with air. Furthermore the DBF device is gastight and is executed to be installed in EX (ATEX) zones.

Biogas, with approximately 60–70% CH₄ and 30–40% CO₂ has different solubility properties from air. The equilibrium between the (partial) pressure in the gas phase and the solubility in the liquid phase is described in Henry's Law. In accordance with Henry's Law the solubility of biogas is approximately 16 times higher than air. At standard conditions this is 291 ml/l for biogas and 17.5 ml/l for air. This is mainly due to the high solubility of CO₂, in fact the solubility of methane is quite similar to oxygen. Another important factor is that the biogas is already in equilibrium between liquid and gas phase, whereas in DAF systems the air concentration in water is not in equilibrium with the air. This means that virtually all biogas which will be dissolved under pressure will be released as gas bubbles upon release of pressure, and therefore biogas can be used for flotation very well. We determined that for a proper flotation of the bio solids, approximately15 l/kg solids is required. Prior to the flotation, the bio solids need to be flocculated. For this purpose a flocculant is added in the pipe reactor upfront the DBF device.

The clarified liquid from the DBF is discharged or fed to a polishing step. In case aerobic treatment is used as polishing, the excess sludge of this treatment is returned to the Aecomix^TM reactor. Figure 1 shows a typical diagram of the Aecomix^TM process.

A full scale plant Aecomix^TM was installed at a chocolate/candy factory. The wastewater volume was 100 m³/d, with a COD concentration varying between 20,000–60,000 mg/l at an average of 37,000 mg/l. The process comprises the following main components:

• Equalisation tank.

• 2 Aecomix^TM reactors of 750 m³ each in series with mechanical mixing.

• Pipe reactor with flocculant dosing.

• DBF.

• Post treatment.

Based on an effective liquid volume of 1,300 m³, the organic loading was 1.8 kg COD/m³/d on average, with a peak load of approximately 4 kg COD/m³/d. The flotation device (shown in Figure 6) was operated at 5 m³/h. The stability of the performance of the Aecomix^TM plant was measured with the ‘Fos-Tac’ (Voß, Weichgrebe and Rosenwinkel). FOS stands for amount of organic acids expressed in mg/l of acetic acid. TAC stands for Total Inorganic Acid, expressed as mg/l CaCO_3. Titration results with Sulphuric Acid to 2 pH points determine the FOS-TAC value (dimensionless) that indicates whether there is a healthy condition in the anaerobic reactor.

Figure 2 shows that there were large variations in the inflow of the Aecomix^TM between 3,600 mg/l and 67,100 mg/l of COD. The FOS-TAC in Figure 3 indicates that the plant is able to handle the shocks very well. Values for FOS-TAC indicating a stable environment are: 0.3–0.4. Lower values indicate a form of under loading and when the value increases above 0.5 it becomes critical on the high load end. Only the FOS-TAC in digester 1 peaks a few times at 0.35, which is actually a good operational value. Generally the FOS-TAC is below 0.2, which indicates that the plant is not fully loaded. Figure 4 shows that the organic loading of digester 1, is generally below 5 kg COD/m³/d. Due to recycling from digester 2 to digester 1, the organic load is more spread over both reactors.

The average COD load was 0.23 kg per kg mixed liquor suspended solids (MLSS) per day, which shows agreement with the other data that the plant is operating in a stable window. Due to the strong variations in the input load, it is also essential for sustaining stable results to operate within this window.

Figure 5 shows that the COD at the outlet of the Aecomix^TM /Dissolved Gas Flotation (DGF) is generally below 1,000 mg/l and varies a bit as a consequence of the high variations in the influent concentrations. Figure 2 shows that the effluent discharged from the DBF device, is achieving high removal percentages for COD, generally well in excess of 95%. Also TSS removal in the DBF is averagely more than 95%.

The flow to the DBF is 5 m³/h. The average MLSS is 6,000 mg/l (6 kg/m³). This means that the solids loading to the DAF is 30 kg/h. The plant operates with 10 l/min gas supply, which equates to 600 l/h and is 9% of the recycle flow. As all or nearly all gas will be released as gas bubbles after the pressure is released, the gas to solids ratio can be based on gas supplied divided by amount of solids/h and is 600 l/h / 30 kg/h = 20 l/kg. As we used a gas to solids ratio of 15 l/kg in our design, there is 30% excess gas available.

The floating sludge is discharged with a dry solids content which varies between 5–9% d.m. For a good performance of the DAF, the dosing of a flocculant is required. The amount of flocculant dosed, influences the effluent quality. For optimum performance in the current setup, the dosage is between 3–6 g/kg dry solids.

In order to evaluate the cost efficiency of this concept a comparison on total yearly costs was made between the Aecomix^TM concept and a traditional concept consisting of full chemical treatment followed by high rate type anaerobic treatment (EGSB).

In the CAPEX the following is considered for the Aecomix^TM:

• Aecomix^TM concept as described above.

• Biological treatment for polishing

• Digestate dewatering by a decanter.

In the traditional concept the following is considered:

• A flocculation-DAF system with 350 mg/l Poly Aluminium Chloride (PAC) 200 mg/l NaOH and 5 mg/l flocculant.

• An EGSB reactor.

• Biological treatment for polishing

• Sludge dewatering by a decanter.

• Digester for DAF sludge

In Table 4 the starting points are given and in Table 5 an overview of CAPEX, OPEX and total yearly costs for Aecomix^TM is presented for this chocolate/candy factory.

Table 5 shows that the total investments for an EGSB concept are slightly higher compared to Aecomix^TM concept. The total yearly cost for the Aecomix^TM concept is with 152,000 EURO(EUR)/year more than 50% lower compared to a traditional system with flocculation/flotation/EGSB reactor. The OPEX savings on chemical consumption, operator attendance and the slightly higher biogas revenues result in this high reduction in total yearly costs for the Aecomix concept compared to a traditional solution.

The Aecomix^TM system, an anaerobic process based on solids retention, is a highly promising alternative for food and beverage plants which have wastewaters with high TSS and/or FOG. It is even more interesting when also factory wastes need to be disposed of. Another promising application will be POME. The Aecomix^TM system provides a single step process solution for different substrates, with a high removal efficiency (on organic matter). It is proven to be a robust process with advantages such as integrated gas storage.

Several full scale plants have shown the effectiveness and suitability of a DBF device as a means of solids-liquid separation after a digestion step. Typically in the Aecomix^TM, the MLSS is relatively low compared to slurry digesters, which makes the DBF an interesting option. Biogas can be used directly as a flotation gas source in the DBF device. The DBF proofs to be an economical method of solids-liquid separation.

A full scale Aecomix^TM reactor treated the effluent of a chocolate/candy factory with a COD concentration varying between 10,000–60,000 mg/l at an average of 37,000 mg/l. The removal for COD and TSS was more than 95% on average. Compared to a traditional set-up with full chemical treatment as pre-treatment before an EGSB type reactor, the Aecomix^TM, has a more than 50% lower yearly costs. This proves that it is an economic feasible option for food and beverage plants to treat wastewater and waste together and produce maximum biogas.