Varec Biogas (Ovivo) vs Smith & Loveless for Digestion

Introduction

One of the most persistent operational headaches in municipal wastewater treatment is the gradual failure of anaerobic digester efficiency due to inadequate mixing and poor gas handling. Engineers often find themselves managing digesters that have lost 30-40% of their active volume to grit accumulation and scum blankets, necessitating dangerous, expensive cleanouts that can cost upwards of $250,000 per tank. The decision on which technology to specify for digester rehabilitation or new construction is critical, often boiling down to a choice between gas injection technologies and hydraulic/mechanical pumping systems.

When evaluating the landscape of established heavyweights, the comparison of Varec Biogas (Ovivo) vs Smith & Loveless for Digestion represents a fundamental engineering decision between two distinct philosophies: unconfined gas mixing combined with comprehensive gas safety (Varec/Ovivo) versus external hydraulic recirculation and mechanical robustness (Smith & Loveless). These manufacturers dominate specific niches within the solids processing train, and understanding where their technologies overlap—and where they diverge—is essential for specification safety.

This article provides a detailed technical analysis for consulting engineers and utility directors. It moves beyond glossy brochure claims to examine the hydraulic principles, maintenance realities, and lifecycle costs associated with these systems. Whether you are retrofitting a 50-year-old fixed-cover digester or designing a high-rate egg-shaped reactor, understanding the specific application fit for Varec Biogas (Ovivo) vs Smith & Loveless for Digestion will prevent costly change orders and long-term operational deficiencies.

How to Select and Specify

Comparing Varec (now under the Ovivo umbrella) and Smith & Loveless (S&L) requires a bifurcated approach. Varec is historically the industry standard for gas safety equipment (flame arresters, PRVs) and gas mixing (bubbler systems). S&L is renowned for pumping systems and hydraulic handling. Therefore, the specification choice is often between Gas Mixing vs. Hydraulic Mixing, or determining if a single-source vendor is required for the entire digestion complex.

Duty Conditions & Operating Envelope

The primary driver for selection is the digester’s physical geometry and the characteristics of the sludge.

• Solids Concentration: Hydraulic mixing systems (typical of S&L designs) generally maintain efficiency up to 5-6% Total Solids (TS). Above this threshold, friction losses in external piping increase exponentially, requiring significantly higher horsepower. Gas mixing systems (Varec/Ovivo) can sometimes handle slightly higher viscosities but are dependent on the compressor’s ability to overcome hydrostatic pressure and sludge rheology.
• Tank Geometry:
  • Cylindrical/Pancake: Both systems work, but floor scour becomes critical. Hydraulic nozzles must be aimed precisely to create a toroidal flow pattern.
  • Egg-Shaped: Often favor draft tube or vertical gas injection to utilize the natural convection of the shape.

• Gas Production Rates: For Varec systems, the available biogas is the mixing medium. In startup scenarios where gas production is low, the system must operate on natural gas or require a specialized startup protocol. Hydraulic systems (S&L) are independent of gas production.

Materials & Compatibility

Corrosion is the defining failure mode in anaerobic digestion headspaces.

• Gas Phase (H2S): Components exposed to biogas must resist hydrogen sulfide corrosion. Varec’s gas safety equipment typically specifies aluminum, 316SS, or specific castings designed for sour gas service. Mixing lances inside the tank (Varec style) are usually 316SS.
• Liquid Phase (Abrasion): S&L systems rely on passing sludge through pumps and piping. Impeller material hardness is critical if the plant has poor headworks/grit removal. Hardened iron or high-chrome alloys are necessary to prevent rapid wear of pump volutes.
• Struvite Potential: This is a major differentiator.
  • Gas Mixing (Varec): Struvite (magnesium ammonium phosphate) tends to precipitate on gas lances and bubbler tubes, especially at the gas/liquid interface. This can plug orifices, requiring the tank to be taken offline for cleaning.
  • Hydraulic Mixing (S&L): Struvite forms in pipelines and pump volutes. Glass-lined pipe or frequent acid washing loops are often required specifications to maintain C-values in the recirculation piping.

Hydraulics & Process Performance

When analyzing Varec Biogas (Ovivo) vs Smith & Loveless for Digestion, the definition of “mixing” differs.

Varec (Gas Mixing): Rely on the buoyancy of gas bubbles to create vertical lift. The specification should focus on SCFM per 1000 ft³ of volume. The mixing energy is derived from the isothermal expansion of gas. It creates “zonal” mixing, which is generally effective for volatile solids reduction but can struggle with heavy grit suspension on the tank floor if floor coverage is inadequate.

S&L (Hydraulic Mixing): Relies on momentum transfer from a nozzle. The specification logic is Horsepower per 1000 ft³ or Turnover Time. S&L typically utilizes non-clog pumps (often wet-well mounted or dry pit) to draw sludge and re-inject it. This provides positive, verifiable movement of fluid but introduces ragging risks at the pump suction.

Installation Environment & Constructability

Space constraints often dictate the winner in retrofit applications.

• Compressor Room vs. Pump Room: Varec systems require a compressor building. The piping running to the tank is relatively small diameter (gas lines). S&L systems require large diameter sludge piping (8″-12″+) and space for pumps. In tight galleries, routing 12″ DIP is significantly harder than 2″ SS gas lines.
• Tank Penetrations: Varec gas systems typically enter through the cover (floating or fixed). S&L hydraulic systems may require core-drilling the tank walls for suction/discharge nozzles, which can be structurally risky in older prestressed concrete tanks.

Reliability, Redundancy & Failure Modes

• Redundancy: Specifications should require N+1 compressors for gas systems or N+1 pumps for hydraulic systems.
• Ragging: S&L pumps, while robust, are susceptible to ragging if screening is poor. Chopper pumps or specific impeller geometries must be specified. Varec gas mixing has no moving parts in the liquid, making it immune to ragging, though the compressors themselves require maintenance (oil changes, valve replacements).

Maintainability, Safety & Access

Safety considerations extend beyond simple equipment failure.

• Gas Safety Integration: Varec provides a holistic “cover-to-flare” safety approach. Their pressure/vacuum relief valves and flame arresters are standard specs. Integrating their mixing system usually simplifies the safety certification of the gas train.
• Maintenance Access: S&L markets “Safe-T-Grate” and external maintenance philosophies. Their systems keep operators out of the tank. Varec systems also keep operators out during normal operation, but internal maintenance (bubbler cleaning) is a major confined space event.

Lifecycle Cost Drivers

• Energy Efficiency: Gas mixing (Varec) historically consumes less power than hydraulic mixing (S&L) for the same active volume, as overcoming hydrostatic head with gas buoyancy is often more efficient than pumping viscous sludge against friction losses.
• OPEX: Hydraulic systems have higher wear parts costs (seals, impellers, wear rings). Gas systems have lower parts costs but potentially higher labor costs if lances foul.

Comparison Tables

The following tables break down the technical differences between the core technologies offered by these manufacturers. Table 1 focuses on the mixing methodology, which is the primary point of divergence when comparing Varec Biogas (Ovivo) vs Smith & Loveless for Digestion. Table 2 outlines the typical application fit for varying plant constraints.

Engineer & Operator Field Notes

Real-world experience often diverges from the theoretical curves provided in submittals. The following sections detail practical insights from field deployments of Varec Biogas (Ovivo) vs Smith & Loveless for Digestion technologies.

Commissioning & Acceptance Testing

Verifying mixing performance is notoriously difficult once the tank is filled.

• Lithium Tracer Testing: This is the gold standard for both Varec and S&L systems. A pulse of lithium chloride is injected, and the concentration is monitored at the withdrawal point.
  • Success Criteria: Look for a “Active Volume” > 90%. If the tracer indicates only 60-70% active volume, you have significant dead zones or short-circuiting.
  • Varec Specifics: Verify that the compressor discharge pressure matches the hydrostatic head calculation. If pressure is lower than calculated, you may have a broken lance (leak) near the surface.
  • S&L Specifics: Verify nozzle velocity. Use a portable Doppler flow meter on the exterior piping to confirm flow rates match the pump curve.

• Temperature Profiling: Install temperature probes at multiple depths. A well-mixed digester should not vary more than 1°F to 2°F throughout the vertical profile. Stratification indicates mixing failure.

Common Specification Mistakes

• Undersizing Compressors (Varec): Engineers often size compressors based on steady-state gas production. However, “burst” or “cannon” modes (sequenced gas injection) require higher instantaneous flow rates. Ensure the VFD and motor are sized for the peak sequencing load, not just the average.
• Ignoring Pipe Friction (S&L): Sludge at 5% solids behaves as a non-Newtonian fluid. Using standard water curves for friction loss in recirculation piping will result in undersized pumps that cannot achieve the required nozzle velocity. A safety factor of 1.5x to 2.0x on friction head is often prudent.
• Mismatched Pressure/Vacuum Relief (Varec): When specifying Varec mixing, ensure the tank’s PVRV (Pressure Vacuum Relief Valve) is sized to handle the maximum instantaneous gas addition from the mixing system plus the peak biological gas production. Failing to account for the mixing gas input can over-pressurize the tank.

O&M Burden & Strategy

• Smith & Loveless: The primary burden is mechanical seals and wear rings. In grit-heavy environments, volute thickness checks should be performed annually using ultrasonic testing. The “wet well mounted” design usually affords good ergonomic access, but expect to change seals every 2-3 years.
• Varec (Ovivo): The compressor is the heart of the system. Rotary vane compressors require vein replacement and rigorous oil changes. The hidden burden is the “condensate traps” in the gas lines. These must be drained daily or automated; otherwise, liquid blocks the gas flow, stopping mixing in that zone.

Design Details & Calculations

When engineering the system, relying solely on vendor recommendations can be risky. Use these design parameters to validate the proposals.

Sizing Logic: Mixing Energy

Regardless of whether you choose Varec Biogas (Ovivo) vs Smith & Loveless for Digestion, the system must meet minimum energy inputs.

1. Hydraulic Mixing (Smith & Loveless Approach)

The standard benchmark is Horsepower per Unit Volume.

• Standard: 0.2 to 0.3 HP per 1,000 ft³ of active volume.
• Velocity Gradient (G): Target G-values between 50 and 80 sec⁻¹.
• Turnover Time: The total tank volume should be theoretically pumped through the mixing system every 20 to 30 minutes.
• Calculation:
  Required Flow (Q) = Volume / Turnover Time
  Ensure nozzle velocity exit is > 20 ft/s to promote momentum transfer.

2. Gas Mixing (Varec Approach)

The benchmark is Gas Flow per Unit Volume.

• Standard: 4.5 to 6.0 SCFM per 1,000 ft³ of active volume (unconfined mixing).
• Floor Coverage: Crucial for unconfined gas systems. Ensure gas lances or floor diffusers are spaced to influence the entire floor area. A typical radius of influence for a single discharge point is 5-8 feet.

Specification Checklist

Include these specific line items in your Division 43 or 46 specifications:

1. Performance Guarantee: Vendor must guarantee >90% active volume as verified by lithium tracer test.
2. Vibration Analysis (S&L): Require factory vibration testing for recirculation pumps at the design operating point (sludge duty, not just water).
3. Spark-Proof Construction (Varec): All in-tank components and gas handling equipment must meet NFPA 820 requirements for the classified area.
4. Material Certifications: 316L Stainless Steel for all wetted gas piping; Hardened Iron (min 400 BHN) for hydraulic pump impellers.

Frequently Asked Questions

What is the main difference between Varec Biogas and Smith & Loveless in digestion?

The main difference lies in their core mixing technologies and product scope. Varec Biogas (Ovivo) specializes in gas injection mixing (bubblers/lances) and provides the complete gas safety train (flame arresters, burners). Smith & Loveless specializes in hydraulic mixing using external centrifugal pumps and nozzle systems. Varec mixes with gas bubbles; S&L mixes with liquid velocity.

Which system is better for scum suppression?

Generally, hydraulic mixing systems (like Smith & Loveless) with dedicated “scum nozzles” are superior for physically breaking up heavy scum blankets. The high-velocity liquid jet can chop and re-entrain grease caps. Unconfined gas mixing (Varec) disrupts the surface, but can sometimes allow scum to accumulate between bubble zones unless specifically designed with draft tubes or surface-level gas lifters.

Can I use Varec gas safety equipment with Smith & Loveless mixing?

Yes, this is a very common configuration. Engineers often specify Smith & Loveless for the sludge mixing/recirculation pumps (for their mechanical reliability) and specify Varec (Ovivo) for the cover safety equipment, waste gas burners, and flame arresters. This “best-of-breed” approach utilizes the strengths of both manufacturers.

How does maintenance differ between the two systems?

Varec maintenance is focused on the compressor room (oil changes, valves) and condensate management in gas lines. The in-tank maintenance is rare but difficult (requires draining the tank). Smith & Loveless maintenance is focused on the pumps (seals, impellers, bearings). While pump maintenance is more frequent, it is performed externally without interrupting the digestion process, which many operators prefer.

What is the typical lifecycle cost difference?

Varec gas mixing systems typically have lower energy costs (lower HP requirements) but may have higher long-term costs associated with cleaning in-tank components if struvite is present. Smith & Loveless systems generally have higher energy costs (pumping viscous sludge is energy-intensive) and higher wear-part costs, but offer greater operational continuity and easier access.

How do I select between gas and hydraulic mixing for a retrofit?

If the existing tank has good structural integrity but limited access for large piping, Varec gas mixing is often easier to retrofit because gas lines are small and flexible. If the tank has a history of grit accumulation, Smith & Loveless hydraulic mixing with floor-scouring nozzles may be preferred to keep the floor clean, provided you can accommodate the large core drills required for suction/discharge piping.

Conclusion

When deciding between Varec Biogas (Ovivo) vs Smith & Loveless for Digestion, the engineer is essentially choosing between two mixing philosophies: the buoyancy-driven efficiency of gas or the kinetic robustness of hydraulics. There is no single “correct” choice for every plant. High-rate digesters with heavy grease loads may benefit from the shear forces of Smith & Loveless hydraulic nozzles. Conversely, large diameter tanks where energy efficiency is paramount may favor the zonal mixing of Varec’s gas injection systems.

The most successful designs often result from a rigorous analysis of the specific sludge rheology and the facility’s maintenance culture. If the operations team prefers external mechanical work over compressor maintenance, the hydraulic solution wins. If the facility is energy-sensitive and has clean sludge (low grit/struvite), gas mixing offers a lower lifecycle cost. Ultimately, the specification must detail the performance criteria—turnover time, active volume, and serviceability—rather than simply copying a vendor’s standard boilerplate.