Application of Covered Anaerobic Lagoons for pre- treatment of Wastewater in red meat and other industries Abstract Keywords: Introduction

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1 Application of Covered Anaerobic Lagoons for pretreatment of Wastewater in red meat and other industries Mitchell Laginestra*, Anthony Allan, GHD Pty Ltd, 211 Victoria Square, Adelaide SA 5000, Australia, Abstract Australian red meat processing plants generate significant volumes of high strength wastewater as part of their normal operations. In the treatment of wastewater, anaerobic lagoons often form one of the first steps, and are a cost-effective process but can suffer from odour emissions. With the awareness of climate change and the wish for companies to reduce the emissions of greenhouse gases, along with an improvement in technology, covered anaerobic lagoons (CAL s) are being investigated. Covered anaerobic lagoons have become popular in the red meat industry in Europe and USA and are also becoming more popular in a number of Australian industries, where land is available. Covered lagoons provide a number of benefits over conventional uncovered lagoons: Opportunity to control odours. Capture of greenhouse gases. Use of biogas to generate electrical energy and thus minimise the carbon footprint of the facility. This paper describes the rationale for covered lagoon technologies. Appropriate design criteria (including depth, cover type, gas removal, pre-treatment, gas cleaning), safety aspects, desludging, downstream treatment requirements and how to operate and maintain anaerobic lagoons are outlined. Keywords: biogas, covered anaerobic lagoon, meat wastewater treatment Introduction Australian red meat processing plants generate significant volumes of high strength wastewater as part of their normal operation. In the treatment of wastewater onsite, anaerobic lagoons often form one of the first steps, which are a cost-effective process but can suffer from odour emissions where natural crusts do not form. Covered anaerobic lagoons (CAL s) have become popular in the red meat industry in the US and other countries, and are also becoming more popular in a number of Australian industries, where land is available. While uncovered ponds are common, the covering provides the opportunity to collect gas, control odours, reduce greenhouse gas emissions, and optimise energy recovery. However the Australian red meat industry remains somewhat apprehensive about covering these lagoons, citing issues such as: Accumulation of greases under the cover and impact on treatment / maintenance if not removed prior. Operational access difficulty and ability to desludge. Safe use or release of the generated biogas. This paper is based on work undertaken for the meat industry association in Australia to improve knowledge and ultimately operational performance of anaerobic lagoons. This includes generation of biogas and usage of this in providing renewable energy. Ponds are used to treat a variety of wastewaters around Australia. The main advantage of pond systems is their simplicity to build and operate, although their non-mechanical aspect means a greater volume (and subsequently area / footprint) is required to treat wastewater than conventional (mechanical) treatment systems. Their ability to achieve significant reductions of contaminants is attributed to their diverse biology and incorporation of aspects

2 of conventional treatment including biochemical reactions and settlement of solids. Different types of lagoons serve different purposes, and the range of operating parameters distinguishes the type and performance. Lagoon types include anaerobic lagoons, which are designed to cater for high organic loading, and are typically absent of DO and contain no significant algal population. Lagoon systems typically comprise a treatment train, which may involve a series of lagoons anaerobic / facultative, aerobic / maturation - to achieve BOD reduction. The appropriate treatment train / series is dependent on organic (BOD) loading. Lagoon systems have been popular with the red meat (and other) industries for treatment of wastewater. Anaerobic lagoons, in particular are attractive for meat processing wastewater, as part of the treatment train, in USA and Australia due to ability to cater for high BOD and low operating costs, and since climate conditions and land availability allow the construction of large lagoons (Johns, 1995). While the majority of lagoons are uncovered, covering of the anaerobic lagoons provides a number of benefits, such as: Opportunity to control odours. Capture of greenhouse gases. Use biogas to generate electrical energy and thus minimise the carbon footprint of the facility. In addition, covering lagoons reduces heat losses with the result of higher microbial reaction rates (Mittal, 2006). Red Meat wastewater Characteristics and Disposal Meat processing wastewater is typically composed of dissolved solids, blood, gut contents, urine and water, with high concentrations of suspended solids, including fat, grease, manure, and undigested feed. This wastewater needs to be treated before being discharged due to potential health concerns and the environmental impact of the discharge. Consequently an effective wastewater treatment train must be installed to deal with the abattoir specific wastewater. Key contaminants of concern are typically BOD over 2,000 mg/l TKN over 200 mg/l Oil / grease - over 500 mg/l In Australia, most meat processing plants are located in rural locations, and many plants reuse their treated wastewater by irrigation of adjacent pasture land. The fodder and soil characteristics allow assimilation of nutrients and organic compounds and to meet sustainable application requirements a nutrient balance is important. For sewer, or other environmental discharges, further reductions of contaminants may be required. Irrigation of effluent in Australia typically requires stringent effluent quality criteria: BOD/SS <100 mg/l Oil/Grease < 25 mg/l Cfu < 1000 No./100 ml To achieve these criteria requires treatment, typically a series of lagoons (normally anaerobic, followed by aerobic) since anaerobic treatment alone will not achieve either criteria (with over 95 % BOD reduction being required). Lagoon Technology Application and Design Anaerobic digestion is a common form of treatment in many countries for a variety of industries, mainly due to low cost and ability to collect odours / gas. Anaerobic ponds require a long detention time to process organic wastes the low solids fermentation system converts volatile solids and soluble BOD to gas, reducing the BOD and the solids content of the wastewater. The pond volume needs to be sufficiently large to reduce the BOD by 90%

3 (7 to 60 days is often regarded as appropriate but dependent on environmental factors). Due to the large scale of processing plants and variety of climates across North America, and even internally within the U.S. there has been no standardization or favouritism of one design or process for treating wastewater. However in Europe there appears to be little development of CAL technology, presumably due to lack of space and advent of more compact treatment systems (constructed reactors, with contained gas storage). As per environmental requirements, however, there are a number of technologies that are applied to meat processing plants - largely mechanical / biological / modular systems. However, anaerobic digestion does feature heavily (since BOD of processing plants is high) but these generally involve constructed concrete reactors rather than lagoons. In Asia CAL s are commonly used for palm oil treatment systems (BOD > 25,000 mg/l). In Asia, Australia and US while there is use of the biogas, the extent however is limited. Europe, it is more developed. The main factors affecting organic removal efficiencies are temperature, retention time, and volumetric loading. Anaerobic pond design is typically based on a generic maximum loading rate of 300 g BOD per m 3 of pond per day (Laginestra and van- Oorschot 2009). However, it should be noted that performance of ponds is very much dependent on temperature, and recommended loading ranges from kg BOD/m 3.d (for o C) (Mara et al 1998). In addition, loading can be affected by geometry, with typical pond arrangement involving a depth of 3 to 5 m, and a rectangular shape of length to width ratio of 3 to 5. Modifications to the wastewater inlet such as the addition of diffusers or horizontal pipes or the incorporation of baffles (Laginestra and van-oorschot 2009), solids recycle or subsurface agitation avoids short-circuiting and thus improves treatment performance. Sludge circulation also enhances the interaction between feed and microbial cells. The yield of biogas from abattoir anaerobic ponds should be in the range 0.25 to 0.5 m 3 /kg COD removed. Odours are often an issue with uncovered ponds, so any move for covering of these types of ponds can achieve both odour reduction and biogas collection. The methane-rich biogas could be used to power a gas engine to generate electricity and the exhaust heat used for absorption refrigeration. Requirements for Covers The lagoon must satisfy a number of design criteria but these are similar to uncovered anaerobic lagoons. The cover must anticipate a number of potential issues and problems and be designed to overcome these. These include issues such as: Stormwater/rainfall ponding on the lagoon cover. Effects of wind and other natural disturbances. Build-up of sludge blanket underneath cover. Accumulation of scum, fats, oils and grease under cover. Need to de-sludge the lagoon. Sizing of biogas off take system. Covers are typically made of polyethylene, and may be trenched to avoid leakage. Safety gas relief is an inherent feature requirement. While some authorities ask for access hatches into the covers, it is more common to remove the trench cover and peel the cover back for desludging. Use and Treatment of Biogas Biogas mainly consists of methane and carbon dioxide, but also contains a number of other constituents which can potentially have adverse impacts on equipment and downstream uses. The three most common uses of recovered biogas typically include (1) consumption as energy-rich boiler fuel; (2) co-generation using a reciprocating gas engine or microturbine to generate electricity and hot water; and (3) flaring in a purpose-built flare.

4 Biogas is saturated with water when produced from the anaerobic wastewater treatment process and can condense when cooled. The presence of water also promotes the corrosivity of the gas components hydrogen sulphide (H 2 S, which can be > 1,000 ppm), carbon dioxide (CO 2 ) and oxygen (O 2 ) (if present). For reciprocating gas engines, water condensation in combustion chambers can wash the lubricating oil off cylinder walls, resulting in higher wear and tear. Water can also accumulate at any low sections of pipe causing biogas flow restrictions, if the piping system is not designed with correct falls and condensate removal. Free water (and condensate) can be easily removed through knock-out pots and drip traps. U-traps and condensate collection sumps are also commonly used. Reciprocating gas engines and microturbines require further water removal which can be achieved through refrigeration of the biogas, which cools the biogas to below the dew point temperature, forcing the water to condense. When the biogas re-heats (either naturally, in a blower, or using a heat exchanger), the relative humidity is reduced. Sulphur dioxide (SO 2 ), formed from hydrogen sulphide (H 2 S) during the combustion process, can lead to the production of sulphurous and sulphuric acid from the reaction with water when combustion exhaust gases are cooled below the dew point. The lubricating oil of gas reciprocating engines can also become contaminated with sulphur and require more frequent changing. Below is a summary of the requirement for H 2 S and water removal for the different biogas use options, based on a review of the quality requirements and an inlet H 2 S concentration of 1,500 ppm. Table 1 Cleaning of Biogas Process Equipment H 2 S Removal (limit) Water removal method Flare No (limit not specified) Yes - Free water removal (e.g. knock-out pot) Boiler No (limit not specified) Yes - Free water removal (e.g. knock-out pot) Reciprocating gas Yes (< 200 ppm) Yes - Drying (e.g. refrigeration) engine Microturbine No (< 5,000 ppm) Yes - Drying (e.g. refrigeration) Key Benefits, Challenges and Issues of CAL s The key benefits of covered anaerobic lagoons when compared to other treatment methods are: Low construction costs. Minimal operating costs. Simplicity of design and operation. Land is more readily available in Australia. The benefits of covered anaerobic lagoons when compared to uncovered anaerobic lagoons are: Reduced odour admissions. Lower hydraulic retention times. Higher possible organic loading rates. Recovery of biogas for generation of energy. Use of biogas for boilers allows CAL s to operate in colder climates. The key challenges and potential issues with operating a CAL (noting there may be some differentiation between treatment and electricity production, where cogeneration is installed) are: Sludge accumulation and desludging. Stormwater/rainwater management. Ensuring consistent inflow and pollutant concentrations.

5 Balancing water quality requirements with biogas production. Ensuring no gas is able to build up within pumps or mechanical equipment. Ensuring gas pressure under the cover doesn t build up too much. Protecting cover from wildlife, plant growth, rips and tears. Proper safety requirements, i.e. fencing, signage and lifebuoys. Scum accumulation. Ensuring appropriate pre-treatment of waste. Designing a flare system able to meet safety and environmental standards for full range Cleaning gas before generating electricity within an engine, to stop H 2 S build up. Operation and Failure of Anaerobic Systems During normal operation anaerobic digestion, will operate at a largely neutral ph, producing gas as a direct result of microbial degradation of organic wastes. Anaerobic treatment does have limitations it can be unstable and involves a long start up. Any decrease in gas generation indicates an issue. The process also can be significantly affected by temperature, with gas production at lower temperatures not as high. Optimum operational ph is While mixing is advantageous it is not always easy in lagoons, but would typically comprise a sump pump with recirculation of sludge. The mechanisms of failure associated with anaerobic wastewater treatment systems are considered to be relatively well documented (and typically relate to high organic loading, leading to operational instability with subsequent inhibition of methanogenic bacteria). However recovery from system upsets is not so well documented. It is however considered as a general rule, that the covered lagoons provide better stability of operation that can be attributed to enclosure from ambient temperature and environmental conditions, and exclusion of air. However, physical issues and desludging are considered to be significant issues for covered lagoons. Examples of failure of anaerobic ponds have been observed and relate to: Organic overload, with subsequent unstable conditions (high VFA s, low ph) impacting effluent quality (high BOD in effluent, with limited BOD reduction over pond) and affecting on ability of downstream processes to adequately produce a good quality effluent. Failure to implement adequate desludging leading to reduced detention time, and ability of system to treat waste. Physical blockages of pipes with subsequent overflows or blockage of gas pipes. Examples of CAL s Red meat There are 2 no. 27 ML lagoons, of 7 m water depth. The cover comprises 2.5 mm HDPE and is anchored via a 600 mm burial trench with 300 mm concrete, and backfilled. The lining skirt is welded to the cover. Gas collection is via a 100 mm diameter perforated pipe around the periphery of the lagoon underneath the cover. There are gas relief valves over the surface of the cover. There is only a single inlet and outlet arrangement. Sludge is removed via bottom fixed pipes (vacuum tanker truck). Refer figure below. The 2 no. covered anaerobic lagoons were commissioned early 2012, to replace previous overloaded uncovered lagoons and provide odour control. While consideration has been given for future power generation from the collected biogas (awaiting outcome of Government grants) the gas is currently flared only (via timer, as pressure sensors are reportedly unreliable, with wind causing fluctuations). Existing pre-treatment, consisting of paunch removal, screening and DAF systems are used. New downstream activated sludge system was also constructed to achieve trade waste discharge criteria. Stormwater is removed from the surface of the lagoon (weighting system, and drainage to a sump).

6 Figure 1 Red meat CAL in NSW Australia, cover is positive pressure Poultry The poultry processing plant has a single unit CAL system discharging treated anaerobic effluent to sewer. The CAL was commissioned in The wastewater system has since been updated to include an aerobic system downstream of the CAL to generate a source of reclaimed water. Features of the CAL include: 22 ML volume, 7 m deep Pre-treatment via grit removal (not fats removal). Multiple inlet manifold via 6 inlet ports. There is underdrainage beneath the membrane line to prevent lifting (associated with high groundwater table). Variable level control in the pond. Pond outlet control valve enables flow balancing. Flow measurement to determine total flow to the sewer. Sludge settled at the outlet end is recirculated to provide mixing and biomass contact with influent (including manual valves to surface cover of inlet end of pond to break up any scum formation). Flaring of gas. Figure 2 Poultry plant CAL in Queensland, Australia, cover is negative pressure

7 Summary of Criteria The table below presents a summary of design criteria appropriate for CAL s. Table 2 CAL Criteria Item Criteria Comment Loading rate kg BOD/m 3.d Dependent on temperature, wastewater characteristics, local factors Depth Minimum 5 m Preferably deeper, dependent on soil conditions, lining, embankment Detention time 8 40 days Dependent on temperature, wastewater characteristics, local factors Pre-treatment requirements Cover arrangement/type Removal of grease, paunch material UV resistant HDPE (minimum 2 mm thick) To prevent crust formation under cover Can be flat, or fixed dome arrangement, but need to allow for stormwater removal. May need multiple stormwater take-off sumps to allow for changing wind direction (and locate away from scum accumulation zone) To provide seal, prevent air ingress. Site specific integral with liner To provide cross sectional flow Trenching of liner / cover Earthen trench, or concrete berm perimeter Inlet/outlet Submerged, multi point arrangement Sludge take-off Sludge pipe To avoid removing cover. Multi point, able to take off from different areas. Could provide mixing/recycle to inlet as integral part. Gas extraction Operational and Maintenance aspects Ring main or multi offtake Daily, weekly checking: Gas production ph BOD/VS reduction Undertake regular: Instrument calibration Equipment servicing Cover integrity checks Gas relief valve check Need to ensure level control, avoid blockage. Pressure of cover to enable auto initiation of gas extraction Typically check feed and gas production on daily basis (if gas has dropped off, check ph and VFA s). Conclusions Lagoons are regarded as effective BOD reduction systems for a range of industries where land is available for construction. Covered anaerobic lagoons are entirely practical and appropriate for the meat industry. While design arrangements are very site specific, there are a number of generic guidelines to enhance operational maintenance aspects, include Multi point submerged inlet distribution, emergency relief valves which are maintainable over the cover, stormwater collection, ability to remove sludge without removal of cover and successful anchoring methodology

8 (suitable for ground conditions and liner). When undertaking the design there is a need to account for: Pre-treatment (to ensure fat and excess solids removal prior to CAL). Rainfall removal (from cover). Appropriate design, incorporating prevention of dead zones, mixing ability, lining and depth. Operational skills to achieve operability and reliability of the treatment system (particular in regard to maximising/optimising gas production). Biogas extraction. Utilisation of the gas and payback expectations. Regular sludge removal. Downstream treatment to achieve effluent quality requirements. Biogas use is entirely appropriate and quality should be considered at the initial stages of concept development. Biogas sampling should be undertaken to identify the concentrations of constituents which could potentially have adverse process and mechanical impacts. Technology options for contaminant removal (viz. water and hydrogen sulphide) should be investigated and assessed considering the specific site preferences and considerations. References DeGarie, C. J. et al., Floating geomembrane covers for odour control and biogas collection and utilization in municipal lagoons. Water Science and Technology, Volume 42, pp Johns, M. R., Developments in Wastewater Treatment in the Meat Processing Industry: A Review. Bioresource Technology, Volume 54, pp Laginestra, M & van Oorschot, R (2009), Wastewater Treatment Pond Systems An Australian Experience, OZWATER, Melbourne, March Mara, D. & Pearson, H. (1998). Design Manual for Wastewater Stabilisation Ponds in Mediterranean Countries. European Investment Bank, Lagoon Technology International Ltd McCabe, B. K., Pittaway, P., Baillie, C. & Harris, P., Monitoring the performance of covered anaerobic ponds in the treatment of abattoir waste, Gold Coast, Australia: SEAg 2011: Diverse Challenges, Innovative Solutions, Sept Mittal, G. S., Treatment of Wastewater from Abattoirs Before Land Application a Review. Bioresource Technology, Volume 97, pp Safley, L. M. & Westerman, P. W., Biogas Production from Anaerobic Lagoons. Biological Wastes, 23(3), pp