Space is tight. Discharge limits are tightening. Traditional activated sludge - reliable for decades - can no longer deliver the effluent quality regulators demand, not without building more tankage you do not have room for. Bio Block offers a different path: retrofit the existing basin with a three-dimensional porous biofilm carrier that multiplies the active biomass without adding a single cubic meter of concrete. This article covers how Bio Block works, what its core advantages are, and what happened when a 20,000 m³/d municipal plant in East China made the switch.
THE CHALLENGE: WHY ACTIVATED SLUDGE IS HITTING ITS LIMITS
Conventional activated sludge (CAS) has been the backbone of municipal wastewater treatment for over a century. It works. But it works within constraints that are becoming harder to meet. Three pressures are converging on plant operators worldwide:
Discharge standards are tightening. China's Grade 1A standard (GB 18918-2002) demands effluent COD below 50 mg/L and ammonia nitrogen below 5 mg/L - thresholds that push CAS to its biological ceiling. Many provinces now enforce even stricter local limits. Europe's Urban Wastewater Treatment Directive is following the same trajectory.
Real estate is finite. Most municipal plants built in the 1990s and 2000s are now surrounded by urban development. There is no adjacent land to expand onto. Any capacity increase must happen within the existing tank volume - and CAS offers little headroom once the mixed liquor suspended solids (MLSS) ceiling of 3,000-4,000 mg/L is reached.
Energy and sludge disposal costs are rising. Aeration accounts for 50-60% of a WWTP's total electricity consumption. Sludge handling adds another 20-30%. As electricity tariffs climb and landfill gates close, the operating economics of CAS deteriorate year by year.
The industry answer to all three problems is the same: pack more biology into the same volume. That is exactly what Bio Block does.
HOW BIO BLOCK WORKS: THE BIOFILM PRINCIPLE
From Suspended Growth to Attached Growth
In a CAS tank, microorganisms float freely as flocs. Their population density is limited by sludge settleability - push MLSS too high, and the secondary clarifier fails. In a Bio Block system, microorganisms grow as an attached biofilm on a three-dimensional carrier matrix. The biomass is retained inside the reactor regardless of hydraulic residence time, decoupling solids retention time (SRT) from hydraulic retention time (HRT). This single change unlocks everything else.
The Carrier Architecture
Bio Block is manufactured from high-density polyethylene (HDPE) or polypropylene (PP) with a controlled three-dimensional open-pore structure. The geometry is not random - it is engineered for three simultaneous functions:
Surface area maximization. The Bio Block 55 type delivers a specific surface area of 220 m²/m³. For context, a conventional aeration tank with MLSS of 3,000 mg/L offers an effective biological surface area of roughly 80-120 m²/m³. Bio Block more than doubles the available habitat per cubic meter. This translates directly to higher biomass concentration - typically 8,000-15,000 mg/L equivalent - without any settling penalty.
Three-phase mass transfer. The open-pore geometry allows water, air, and biofilm to coexist in intimate contact. Dissolved oxygen and soluble substrate diffuse into the biofilm from the bulk liquid while metabolic byproducts diffuse out. The tortuous flow path through the carrier block creates micro-turbulence that continuously refreshes the boundary layer - the rate-limiting step in any biofilm system.
Self-regulating biofilm thickness. As the biofilm grows thicker, shear forces from aeration and liquid flow slough off the outer layers. This prevents the diffusion limitation that cripples overly thick biofilms. The result is a biofilm that self-maintains at an optimal thickness of 150-300 microns - thick enough for high activity, thin enough to avoid anaerobic dead zones.
Microbial Stratification and Nutrient Removal
A mature Bio Block biofilm develops distinct functional layers. The outer aerobic layer (0-100 microns) hosts heterotrophic COD degraders and autotrophic nitrifiers - Nitrosomonas and Nitrobacter - that convert ammonia to nitrate. Deeper within the biofilm, where oxygen has been consumed by the outer layers, an anoxic zone forms. Here, facultative denitrifiers use nitrate as the terminal electron acceptor, reducing it to nitrogen gas. A single carrier block thus achieves simultaneous nitrification-denitrification (SND) - a process that in CAS requires separate aerobic and anoxic zones with internal recirculation pumps.
For phosphorus removal, Bio Block provides attachment surfaces for phosphate-accumulating organisms (PAOs). While the biofilm mode cannot fully replace anaerobic-aerobic cycling, it significantly enhances biological phosphorus uptake when combined with a short anaerobic selector zone upstream - an arrangement that delivers effluent total phosphorus below 0.5 mg/L without chemical dosing in most municipal applications.
CORE ADVANTAGES OF BIO BLOCK
| Advantage | What It Means Technically | What It Means for Your Plant |
| High Treatment Efficiency | 220 m²/m³ specific surface area supports biofilm concentrations equivalent to 8,000-15,000 mg/L MLSS; COD removal rates consistently exceed 90% | Meet tightening discharge standards without expanding tank volume or adding tertiary treatment stages |
| Strong Shock Load Resistance | Stable biofilm structure buffers against hydraulic and organic load fluctuations; attached biomass cannot wash out during flow surges | Maintain compliance during rainy season peaks, industrial discharge events, and seasonal loading variations |
| Low O&M Cost | HDPE/PP carrier service life exceeds 15 years; open-pore structure resists clogging; no media replacement cycle | Reduce long-term operating expenditure; eliminate the media replacement downtime common with older carrier designs |
| Wide Application Range | Effective across municipal sewage, industrial wastewater (textile, food processing, chemical), and aquaculture recirculation systems | Standardize on a single carrier technology across multiple treatment lines and applications |
| Footprint-Neutral Retrofit | Filling rate of 55% of effective tank volume; existing aeration diffusers and tank structures are retained | Upgrade capacity and effluent quality without purchasing land, pouring concrete, or interrupting plant operations |
CASE STUDY: MUNICIPAL WWTP UPGRADE IN EAST CHINA
Project Background
The plant serves a mid-sized city in East China with a design treatment capacity of 20,000 m³/d. The original process was a conventional activated sludge configuration - primary sedimentation, aeration basin, secondary clarifier - commissioned in 2008. For over a decade, it met the then-applicable Grade 1B discharge standard reliably. But in 2022, the local environmental protection bureau mandated a shift to a stricter local standard requiring effluent COD below 30 mg/L and ammonia nitrogen below 1.0 mg/L - roughly equivalent to China's emerging quasi-Class IV surface water criteria. The plant was not meeting these limits, and there was no space to build additional treatment stages.
Transformation Design
The engineering team evaluated three options: extending the aeration basin (rejected due to land constraints), adding a tertiary MBR stage (rejected due to capital and operating cost), and retrofitting the existing aeration basin with Bio Block carriers (selected). The retrofit scope was deliberately minimal to keep the plant running throughout construction:
Carrier selection: Bio Block 55 type, manufactured from virgin HDPE with a specific surface area of 220 m²/m³. This model was chosen to provide sufficient surface area for the target effluent quality while maintaining hydraulic flow through the carrier bed.
Filling strategy: 55% of the effective aeration tank volume was filled with carrier media. Higher filling ratios (60-67%) are technically feasible but were avoided here to preserve aeration mixing efficiency and prevent carrier accumulation at the tank outlet. Stainless steel retention screens with 12 mm openings were installed at the tank outlet weir to contain the carriers.
Aeration adjustment: The existing fine-bubble diffuser grid was retained. Aeration intensity was adjusted downward from 4.5 m³ air/m³ wastewater to 3.2 m³ air/m³ wastewater, reflecting the higher oxygen transfer efficiency achieved when biofilm carriers break up coarse bubbles and extend bubble residence time through the carrier bed.
HRT reduction: Hydraulic retention time was shortened from 12 hours to 8 hours. This was not an arbitrary decision - it was the calculated outcome of doubling the effective biomass concentration. With more biology in the tank, each cubic meter of wastewater needs less contact time to achieve the same degree of treatment.
The entire retrofit was completed in four weeks, during which the plant operated at reduced capacity on a bypass arrangement. No new concrete was poured. No additional land was acquired.
PERFORMANCE DATA: SIX-MONTH MONITORING RESULTS
The following data represents monthly average values collected over a six-month post-retrofit monitoring period. The plant was operating at an average daily flow of 18,500 m³/d during this period, close to design capacity.
| Parameter | Influent (mg/L) | Before Upgrade (mg/L) | After Upgrade (mg/L) | Removal Rate |
| COD | 280-360 | 50-65 | ≤25 | ≥92% |
| Ammonia Nitrogen | 25-35 | 8-12 | ≤0.5 | ≥98% |
| Total Nitrogen | 35-45 | 15-20 | ≤10 | ≥78% |
| Total Phosphorus | 3-5 | 0.8-1.2 | ≤0.3 | ≥93% |
Three numbers deserve particular attention. First, the ammonia nitrogen removal rate of over 98% confirms that nitrifying bacteria - notoriously slow-growing and sensitive organisms - established and maintained a robust population on the Bio Block carriers. This is critical because nitrification is often the rate-limiting step in municipal treatment upgrades, and suspended-growth systems require SRTs of 8-15 days to retain nitrifiers. The Bio Block biofilm achieves this effortlessly by immobilizing the nitrifiers on the carrier surface.
Second, the total nitrogen reduction to below 10 mg/L without a separate anoxic zone or internal recirculation loop validates the simultaneous nitrification-denitrification mechanism described earlier. The plant achieved denitrification in the same tank where nitrification was occurring - a result that would be thermodynamically impossible in a fully mixed suspended-growth system.
Third, phosphorus removal exceeded 93%. This required a small upstream anaerobic selector (volume approximately 8% of the aeration tank), but no metal salt coagulant addition. The PAOs enriched on the carrier surfaces were sufficient to drive luxury phosphorus uptake when exposed to the anaerobic-aerobic cycle.
COMPREHENSIVE BENEFITS: BEYOND EFFLUENT QUALITY
The Bio Block retrofit delivered benefits across four dimensions - effluent quality, capacity, operating cost, and resilience - that together make the business case for this technology compelling.
Effluent Quality
Every parameter met the new, stricter local discharge standard with a comfortable margin. Compliance was not borderline - it was robust. The plant operator no longer faces the anxiety of watching COD and ammonia values creep toward the limit during high-load events.
Capacity Increase
By shortening HRT from 12 to 8 hours while maintaining - and improving - treatment performance, the plant effectively gained approximately 30% additional treatment capacity. This was achieved without expanding a single tank. For a plant that had been operating near its hydraulic limit, this headroom is strategically valuable as the city continues to grow.
Operating Cost Reduction
Sludge production decreased by approximately 35% compared to the pre-retrofit baseline. This is a direct consequence of the biofilm mode: attached-growth systems operate at higher SRTs, which drives the biomass further into the endogenous respiration phase where net cell yield is lower. Less sludge means lower polymer consumption for dewatering, fewer truckloads to landfill, and reduced disposal fees.
Energy consumption per cubic meter of treated water dropped by approximately 16%. The primary driver was reduced aeration intensity - the Bio Block carrier bed enhances oxygen transfer efficiency by increasing bubble residence time and creating local turbulence that accelerates interfacial mass transfer. A secondary contribution came from eliminating the internal recirculation pumps previously used for denitrification, since SND now occurs within the biofilm.
Operational Resilience
During the rainy season, when influent flow surged and pollutant concentrations swung widely, the Bio Block system maintained stable effluent quality. The attached biofilm simply does not wash out - a fundamental advantage over suspended-growth systems where a hydraulic surge can strip the aeration basin of biomass in hours. The plant operator reported that the system "recovers within one HRT cycle" after a shock event, compared to days or weeks for the previous CAS configuration.
IS BIO BLOCK RIGHT FOR YOUR PLANT?
Bio Block is not a universal solution. It is specifically well-suited to plants facing one or more of the following conditions:
Stricter discharge standards that the current activated sludge process cannot meet without tertiary treatment. Bio Block can close the gap within the existing biological stage, often eliminating the need for downstream polishing steps.
Capacity constraints without expansion space. If your plant is hydraulically or organically overloaded but has no room to build, a Bio Block retrofit can increase volumetric treatment capacity by 25-40% by raising the effective biomass concentration.
High sludge disposal costs. The 30-40% reduction in waste sludge production translates directly to lower operating expenditure, particularly in regions where landfill tipping fees and transport costs are significant.
Variable industrial or seasonal loads. The biofilm's inherent resistance to shock loads makes Bio Block an excellent choice for plants that receive intermittent industrial discharges or experience large seasonal flow and load variations.
Plants considering Bio Block should evaluate carrier type and filling ratio through a site-specific process calculation that accounts for influent characteristics, target effluent quality, available tank volume, and existing aeration capacity. A pilot trial at 5-10% of full scale is recommended for first-time adopters to confirm the design parameters before committing to a full retrofit.
CONCLUSION
Bio Block as an efficient biofilm carrier material has demonstrated clear advantages in improving treatment efficiency, enhancing shock load resistance, and reducing operating costs. The East China municipal plant case study provides a data-backed reference point: 92% COD removal, 98% ammonia nitrogen removal, 35% sludge reduction, 16% energy savings, and 30% effective capacity increase - all without adding a single cubic meter of concrete.
For municipal WWTP operators facing the triple pressure of tighter standards, space constraints, and cost control, Bio Block represents a technically mature and economically viable upgrade pathway. It is not experimental technology. It is not a laboratory curiosity. It is a field-proven solution with thousands of installations worldwide - and the performance data to back it up.
Planning a WWTP Upgrade or New Biofilm Installation?
Juntai supplies Bio Block carrier media in multiple specifications (Type 35 through Type 65) with specific surface areas from 120 to 350 m²/m³. We provide complimentary process sizing support - including filling ratio calculations, aeration adjustment recommendations, and retention screen design - based on your specific influent data and treatment targets. Contact us to discuss your project.
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