Wastewater Treatment Pond Systems An Australian Experience

Transcription

1 Paper Number: 082, Stream Theme WWSP Wastewater Treatment Pond Systems An Australian Experience Mitchell Laginestra, GHD, Robbert van Oorschot, GHD, robbert_van KEYWORDS Lagoon wastewater treatment, anaerobic pond, facultative pond, aerated pond ABSTRACT Wastewater treatment through lagoon based systems to undertake reduction of BOD and other contaminants essentially encapsulates natural systems yet in a controlled manner. Although taking a much larger footprint than mechanical / constructed bioreactor systems, they have specific uses, are considered sustainable, provide reasonable treatment and are generally prevalent in rural applications. They play a particular role in treatment of intensive agricultural industry wastewater, but are also used for treatment of municipal sewage. Performance of lagoon based systems is varied and is dependent on a range of factors:» Type of pond (anaerobic, facultative, aerobic, aerated, maturation)» Loading and wastewater characteristics» Climate» Arrangement. In addition, based on loading and general performance, odours can be a significant issue, and presents a challenge to operators of pond systems, particularly with encroachment of residential and semi rural communities (into previously industrial or non permanent inhabited areas). This paper outlines issues, applications and maintenance requirements for ponds, and makes some suggested design improvements and looks at general design and operational aspects based on actual pond performance. INTRODUCTION Ponds are used to treat municipal and industrial wastewaters in a variety of applications around Australia. The main advantage of these type of 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 treatment systems. Their ability to achieve significant reductions of contaminants is attributed to their diverse biology and incorporation of aspects of conventional treatment including biochemical reactions, settlement of solids, and disinfection. Different types of ponds serve different purposes, and the range of operating parameters distinguishes the type and performance. Ponds are distinguished largely by the dissolved oxygen (DO) of the layers within the ponds, which in turn, is dependent on the loading of the pond system. Pond types include:» Anaerobic ponds, which are designed to cater for high organic loading, and are typically absent of DO and contain no significant algal population. They typically involve long detention times and are deeper than other ponds due to the need to exclude oxygen;» Facultative ponds, which incorporate two different operating modes, aerobic at the surface and, with the settlement of sludge, anaerobic at the base of the pond. These ponds are typically shallower than anaerobic ponds.» Aerobic ponds, which are shallow to allow algal development and receive lower solids / BOD loading.

2 » / Oxidation ponds, which essentially are used for polishing of effluent, and are shallow to allow for ultraviolet light penetration and subsequent disinfection. There are also mechanically aerated ponds, which can be deeper than naturally aerobic ponds, since the aeration will reach the lower layers. Evaporation ponds are also employed where effluent may be problematic to dispose of, for example with high salinity. Theses are typically very shallow (less than 1.5 m) with volumetric requirements being dependent on environmental factors (temperature, humidity and rainfall). Pond systems typically comprise a treatment train, which involves a series of ponds anaerobic / facultative, aerobic / maturation to achieve BOD reduction, (perhaps nutrient reduction) and pathogen reduction prior to transfer to the environment (irrigation re use or disposal to receiving waters). The appropriate treatment train / series is dependent on loading. In addition, it is possible to overcome shortcomings of ponds oxidation capacity by installation of mechanical aeration which helps in DO as well as mixing ability / prevention of short circuiting. A key aspect in considering ponds for treatment of wastewater is that, despite the significant footprint, they are regarded as a sustainable technology. Pond systems offer:» Low energy consumption compared to more conventional systems;» Oxygenation of the upper water layer via movement of air and natural wave action;» Solar / powered aeration via algal respiration;» Natural ph buffering via carbonate / bicarbonate system;» Natural nutrient uptake and reduction» Solar induced disinfection; and» Biogas generation from anaerobic ponds (where they are covered and gas collected). The disadvantage of pond systems is the inability to significantly remove nutrients. However, some reduction over ponds is well documented and is believed to be associated with volatilisation of ammonia, algal assimilation in biomass, and possibly biological nitrification / denitrification. Nitrification can occur intermittently and can be unpredictable for long lengths of time (particularly during cold periods), and is often attributed to variable oxygen levels, temperature and ph (and likely low numbers of nitrifying bacteria). However, nitrification / denitrification is thought to be a major N removal path in facultative ponds during periods of high algae abundance. The high photosynthetic activity probably raises DO and ph. Denitrification is likely to occur at the lower depths of facultative ponds. Phosphorus uptake is also reported via algae assimilation and precipitation with metal salts. Removal efficiencies of up to % N and % P have been reported in facultative lagoons, (Shilton, 2005). Lower reduction efficiencies are reported in maturation ponds.. Release of nutrients from decomposition of accumulated sludge and resolubilisation form sediments sediment can reduce the overall efficiency of nutrient removal. FACTORS INFLUENCING PERFORMANCE OF PONDS Key design criteria for pond based systems typically consists of organic loading rate (areal and volumetric, dependent on the type of pond) and detention time. There are also a number of key physical design features, which can affect performance:» Depth (which needs to suit the operating conditions for the pond);» Shape and layout arrangement (length to width and inlet / outlet orientation) which dictate plug flow treatment to avoid short circuiting)» Wastewater characteristics» Sludge accumulation (period between clean out) and» Environmental factors (temperature, sunlight, rainfall and wind velocity) Any design for treatment of wastewater needs to take into account the local site specific factors. As with any treatment process system, ponds are affected by changing conditions

3 and success of treatment performance is dependent on enabling development of suitable biological conditions and contact with the contaminants contained in the wastewater. The reported main typical issues for ponds include:» Lack of mixing;» Sludge accumulation or sludge rafting;» Advent of dead zones, which can lead to short circuiting;» Screenings and grease accumulation;» Excessive algae (minimising disinfection ability);» Apparent Increase in solids over ponds either due to significant algal solids or sludge accumulation and release with changing localised velocities or incorporation of mechanical aeration;» Construction issues seepage or leakage development if liner is not properly installed / compacted or poor choice of materials or rupture. REVIEW OF POND SYSTEMS General Table 2 below presents some operating performance indicators and key criteria for a few operating ponds both municipal and industrial applications. However, firstly, we have considered what expectations there are of pond performance and Table 1 presents a short review (from Shilton, 2005) of typical design equations for ponds. For performance of wastewater stabilisation ponds, literature and general experience indicates:» BOD reduction of 50 up to 90 % for anaerobic ponds is possible (but is dependent on loading, recirculation, and noting that anaerobic ponds are typically used to treat high BOD wastes, > 2000 mg/l). Typically no nutrient and limited solids reduction is expected from anaerobic ponds;» BOD reduction between 70 and 85% across facultative and aerobic ponds may be achieved (including mechanically aerated ponds);» Effluent suspended solids mg/l is achievable from facultative ponds;» BOD reduction between 60 and 80% across aerobic maturation ponds may be achieved» Effluent suspended solids mg/l from aerobic maturation ponds may be achieved. Table 1 Pond Design Equations Loading Equation Parameters Criteria / Conditions Anaerobic lagoons Volumetric loading rate (kg/m 3.d), λ V = X i Q / 1000 V facultative lagoons Surface loading rate (kg/ha.d), λ s = 350 ( T) T 25 maturation ponds Surface loading rate (kg/ha.d), λ s = X i D / 100 θ Xi = influent BOD, mg/l Q = influent flow m 3 /d V = lagoon volume, m 3 T = air temperature of coldest month D = depth, θ = detention time λ V = 0.1 (for T < 10), λ V = 0.1 (20T 100) (for T=10 20), λ V = 0.1(10T+100) (for T=20 25), λ V = 0.35 (for T >25) λ S = 80 (for T < 8), λ S = 350 (T > 25) θ pond 1 = 3 5 (for T > 10) but typically to achieve disinfection Pond Case Studies A number of random pond systems have been reviewed and are summarised in the table below.

4 Effluent disposal from each of the pond systems is via a range of avenues, including surface waters, general re use, sewer discharge and agricultural application. Table 2 Pond Performance Review WWTP Climate Type of pond Municipal WWTP s Alice Springs Arid (different pond sets) Facultative Facultative Berrimah Tropical Facultative Katherine Hot Facultative Mount Barker (prior to upgrade) Mild Facultative, with supplemental aeration Current loading 137 kg/ha.d 39 kg/ha.d 87 kg/ha.d 36 kg/ha.d HRT, kg/ha.d kg/ha.d, kg/ha.d kg/ha.d, kg/ha.d 20 Performance BOD / TN / TP % reduction 63, 0, 33 40, 20, 35 44, 0, 0 14, 26, 3 86, 64, 65 SS in effluent / algae induced or not 97, no 85, yes 108, no 73 mg/l yes 238 (no) 53, 50, (yes) mg/l 79, 40, mg/l (no) 0, 30, (yes) mg/l 79, 30, mg/l (yes) Pond No. and suggested loading based on Temp Multiple (150 kg/ha.d) Multiple (150 kg/ha.d) (300 kg/ha.d) Multiple (350 kg/ha.d) (but baffled) (200 kg/ha.d) Namatjira Warm, dry Facultative 16 kg/ha.d , 70, , yes (45 kg/ha.d) Palmerston Tropical Facultative 99 kg/ha.d 86 kg/ha.d 38, 13 65, 60, 15 27, 0, 2 150, no 120, yes (300 kg/ha.d) Multiple Penola Mild Facultative 16 kg/ha.d 90 88, 39, 9 50, yes (20 kg/ha.d) Industrial WWTP s Poultry Hot, Anaerobic d 79, 25, humid (covered) kg/m 3.d mg/l, no (0.05 kg/m 3.d) Winery Mild Aerated 65 d 90, 75, mg/l during vintage 85 d 50, 10, 50 Meat processing Mild / temperate (supplementary aeration) Anaerobic (uncovered) Aerated 0.09 kg BOD/m 3.d) 260 kg BOD/ha.d vintage 0.15 kg BOD/m 3.d 0.19 kg BOD/m 3.d) , 3, 5 79, 4, mg/l yes 840 mg/l, no 500 mg/l, yes (0.1 kg/m 3.d) (200 kg/ha.d) (0.05 kg/m 3.d)) (0.1 kg/m 3.d) Review In reviewing the performance of the above ponds, the following observations are made:» Facultative and ponds are all m deep,» Anaerobic ponds were 3 5 m deep,» Mechanically aerated ponds were typically 3 m deep,» The municipal plants all less than 50,000 equivalent population loading,» Areal organic loading, while obviously a good indicator, does not necessarily provide good indication of performance in general without considering the other factors such as arrangement,» Multiple ponds seemingly provide enhanced performance

5 » Sludge accumulation and localised velocities (as well as mechanical aeration) can result in high solids throughput to other lagoons,» Some nutrient reduction is possible with lagoons, although this is considered to be not consistent,» Of the industrial treatment ponds anaerobic treatment can provide good reduction provided sufficient detention is integrated into the design,» Mechanical aeration is appropriate to reduce BOD and area (i.e., can enable greater areal loading),» Nearly all lagoons involve very little operation / maintenance (and some seemingly suffer from that including infrequent desludging),» Some ponds exhibit no reduction in some contaminants (an increase is occasionally indicated), which may be due to sampling inconsistencies, but more likely to be associated with sludge accumulation and washout as well as algal solids. The latter is particularly the case with municipal ponds (high availability of nutrients) and algae in the final pond is considered to be a significant issue. POND OPERATION Ponds do require some operational control, and monitoring is needed to assess performance, which helps to indicate, in conjunction with visual observations, when cleaning is required. Sludge accumulation / rafting can lead to odour generation, and this is common with ponds from time to time. Remoteness from residential receptors is no guarantee of odour free operation. High loading can accelerate a pond to this condition, and holiday peak loading or operation during high production for industrial facilities are common issues, which an operator must deal with. Vigilance in seepage or overflows (as a result of pipe blockages) must also be checked on a regular basis. DESIGN OF PONDS Design Criteria In reviewing the performance of ponds in general, provided loading is not excessive, BOD reduction is considered reasonable. Nutrient reduction is also possible, but is generally regarded to be inconsistent. The SS from the pond effluent is highly variable, and generally impacted by algal solids or sporadic release of accumulated solids (which demonstrates that, despite general operator perceptions, ponds do need regular desludging). Some possible loading criteria based on this review are outlined below. Table 3 Suggested Loading Criteria for Municipal Ponds Environmental Surface loading Detention time () conditions (kg BOD /ha/d) Facultative ponds Cold seasonal climate Temperate to warm Hot / Tropical ponds Cold seasonal climate Temperate to warm Hot / Tropical Table 4 Suggested Loading Criteria for Industrial Ponds Environmental Organic Loading Detention time () conditions (kg BOD / m 3. d) Anaerobic Ponds Cold seasonal climate Temperate to warm

6 Environmental Organic Loading Detention time () conditions (kg BOD / m 3. d) Aerated ponds Cold seasonal climate Temperate to warm As noted above, meeting the above loading does not necessarily guarantee a success in performance. Rather the loading in conjunction with arrangement and condition of the pond will provide an indicator of expectation for effluent quality. General design features and suggested arrangements gleaned from existing installations and sourced from the references listed at the end of this paper are summarised in the section below. Key design features The final arrangement of ponds in treating wastewater, will, to a large extent, be site specific. This is particularly relevant for environmental factors and wastewater characterisation, but also relates to soil type (which impacts on liner design, and suitability of irrigation), and arrangement of pond layout. There are, however, basic design aspects, which should be take into account to maximise performance. Some of these features may be retrofitted to existing lagoons, but in the main, it is important to integrate good design at the start. Key design features include:» length to breadth ratio (usually a minimum of 2 :1, but can incorporate baffles for facultative / aerobic to have greater ratios: 3 5 :1).» Enhanced inlet / outlet arrangements (inlet diffuser or horizontal pipe) located outlet out of zone that is main flow path or incorporating baffles / flow deflectors» slope of embankments (3:1 internal; and 2:1 external dependent on soil)» Minimum of 500 mm freeboard.» Depths: Anaerobic ponds min depth = 3 > 5 m Aerated ponds min depth = 3 m ponds depth = 1 2 m.» liner material is always required, either but compacted clay of minimum depth 0.3 m or polyethylene / geotextile with properly prepared underlay.» multiple ponds are likely to provide improved effluent quality through minimising short circuiting.» Covering of anaerobic ponds not only provides opportunity for odour control and gas collection (with subsequent use of biogas) but also improves operating by reducing air impact. Need for Preliminary Treatment Hardly any pond systems in Australia incorporate preliminary treatment, apart from the septic effluent collection / drainage schemes, which rely on the septic tank installation at individual houses to remove screenings and grit. Of the ponds reviewed in Table 2, only 2 of the industrial applications incorporated screening. The lack of preliminary treatment has resulted in a number of issues at pond systems notably the appearance of screenings in accumulated sludge mats, and discharge of offensive material to the environment. Figure 1 illustrates some of the issues associated with lack of screening, and the need for regular cleanout of lagoons. Dependent on location (and catchment area) grit can be an issue accumulating adjacent to inlet points and reducing detention time.

7 Figure 1 Screenings accumulation in sludge mat in a facultative pond It is considered that as a minimum, screening should be implemented prior to treatment by pond systems. Uprating of Pond Capacity by Implementation of Changes Upgrading of existing ponds is often not easy (unless they are extended) and implementation of baffling or inlet / outlet modifications can involve complete emptying. Improvements (if deemed required) such as baffling, recirculation, screening and programmed desludging would not be regarded as providing increased capacity for ponds in terms of capacity, but would rather be expected to marginally improve contaminant reduction performance. Desludging is required to remove accumulated solids, while this may not extend the theoretical organic loading capacity, it will potentially increase hydraulic capacity by enabling a greater actual detention time (more volume is available which will theoretically allow greater storage and treatment of the sewage). However, short circuiting can impact on this, and it may mean that desludging will have a lesser effect than theoretical. Aerobic ponds will, however, benefit from having mechanical aeration. So, any shortfall in detention or organic treatment capability may be overcome through providing increased oxygenation capacity. This may be readily accessible, since an aspirator system can be readily installed on floating pontoons and tied via cables and anchored. The are however tow considerations which need to be taken into account with this:» Embankment walls of ponds may require stabilisation (rocks, tyres may be used),» air stream should be angled to avoid impingement of the base of shallow pondage. In calculating the required aeration for aerobic ponds, the oxygen requirements from the BOD loading needs to be taken into account, but there also needs to be an assessment of the ability of pond to provide oxygenation via natural processes (this will involve a review of the extent of loading). For complete mix aerated ponds, the BOD will dictate the actual oxygenation required. Other upgrading of pond systems may involve post treatment, for example, Mount Barker ponds are followed by dissolved air flotation (DAF) and microfiltration systems, prior to discharge to wetlands to produce Class A effluent for re use. Also, Alice Springs has recently installed a DAF system to upgrade effluent quality from the ponds. In considering major upgrades, Western Treatment Plant in Werribee Melbourne is probably the ultimate. The issue here was inconsistent nitrogen reduction. Part of one of the lagoons was converted to provide a more conventional approach to wastewater treatment via an activated sludge plant (Modified Ludzack Ettinger) to remove nitrogen. However, the process involves discharge back to the lagoons to utilise their assimilative capacity, and trials have also been undertaken on introducing an internal pond recycle

8 with the mechanical plant (known as the PETRO process). This reportedly promotes conditions for micro flocculating algae to form, which are removed in the clarifier tanks. CONCLUSIONS Pond systems are widely used in a number of applications. Where there is land available, they are an appropriate technology, approaching sustainable operation (and are regarded as a controlled encapsulation of the natural environment). Their low cost and simple operation make them a very attractive option for treatment of a wide variety of wastewaters. There are, however, a number of criteria and design aspects, which must be adhered to maximise performance. BOD is typically removed to a significant extent (over 80 % in some cases). Nutrients can also be removed but this is largely inconsistent, particularly during winter. In addition, despite widely held perceptions, operational control (including monitoring) and regular desludging is needed, although there are significantly less requirements than conventional systems. Addition of mechanical aeration provides greater control with lower footprint there is a trade off. The cumulative treatment effect of multiple ponds in series helps to achieve significant BOD reduction, but does not necessarily improve SS reduction for pond systems, which are impacted by algal solids. In areas that utilise pond effluent for irrigation of agricultural application, without the need for stringent disinfection criteria or nutrient reduction, then ponds are considered to be an ideal approach for wastewater treatment. If circumstances dictate, pond effluent can be upgraded via implementation of downstream processes to remove residual / algal solids and contaminants to achieve a high effluent quality. Key opportunities for upgrading and operational improvements for existing pond systems are regarded as:» Inclusion of screening to prevent downstream issues;» Implementation of a regular desludging program to minimise accumulation of solids;» Reconfiguration of inlet / outlet or installation of baffles to prevent short circuiting and reduce occurrence of dead zones» Installation of mechanical aeration to improve oxygen transfer, pond mixing and reduce BOD concentrations. REFERENCES EPA (1997), South Australia Pond Guidelines EPA (1996) Design and Management of Tasmanian Sewage Lagoon Systems Laginestra, M & Berzins, A (2006) Mt Barker Wastewater Treatment Lagoon Upgrade Achieving Suitable Effluent Quality For Re Use. Enviro 2006 (Melbourne). Power and Water Corporation (2006) Annual Report Wastewater Treatment, Reuse and Discharge Metcalf and Eddy, Wastewater Engineering, Treatment, Disposal, Reuse, Third Edition, 1991 Shilton, A (ed.) (2005), Pond Treatment technology. Integrated Environmental technology Series, IWA Waste Stabilization Ponds, Earnest F. Gloyna, World Health Organisation, Geneva, 1971.